fix(insights): align BD-Zielbereich delta math with comparison window

The tile computed `bp30 - bpAll` regardless of which baseline the
user picked under Settings → Dashboard, while rendering a caption
keyed on `comparisonBaseline` ("vs. last month" / "vs. last year").
The math therefore lied to the user. Every other trend tile uses
`tileCompareDelta()` against period-aligned `avg30LastMonth` /
`avg30LastYear` baselines.

Add `priorMonth` (now-60d…now-30d) and `priorYear` (now-395d…
now-365d) windows to `computeBpInTargetWindows()`. Ship the pcts on
the analytics envelope as `bpInTargetPctPriorMonth` /
`bpInTargetPctPriorYear`. Route the dashboard's `bpCompareDelta` to
subtract the prior window matching the user's baseline; fall back
to null for `none`.

Three new bp-in-target unit tests (priorMonth, priorYear, null
windows) and two insights-polish guards (envelope contains the
fields, page no longer subtracts `bpAll`).

Closes Code-H2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/__tests__/insights-polish.test.ts

@@ -181,6 +181,40 @@ describe("v1.4.19 A3 — chart-token leak hardening", () => {
   });
 });
 
+describe("v1.4.22 W5 reconcile (Code-H2) — BD-Zielbereich tile compareDelta uses period-aligned baseline", () => {
+  /**
+   * Up to v1.4.22 W4 the BD tile computed `bp30 - bpAll`
+   * (last-30-days minus all-time) regardless of which window the user
+   * picked under Settings → Dashboard. The caption still rendered "vs.
+   * last month" / "vs. last year" via `comparison.captionLastMonth` /
+   * `comparison.captionLastYear`, so the user read a sentence whose
+   * numerator was actually a 30d-vs-all-time delta. Every other tile
+   * routes through `tileCompareDelta()` with `summary.avg30LastMonth`
+   * / `summary.avg30LastYear` for honest period-aligned math.
+   *
+   * This test pins that the dashboard now subtracts the prior-period
+   * window (priorMonth / priorYear) instead of all-time.
+   */
+  it("subtracts bpInTargetPctPriorMonth when comparisonBaseline=lastMonth", () => {
+    const src = load(DASHBOARD_PATH);
+    // The fix introduces `bpComparePrior` keyed by `compareBaseline`.
+    expect(src).toContain("bpInTargetPctPriorMonth");
+    expect(src).toContain("bpInTargetPctPriorYear");
+    // The all-time shortcut from v1.4.22 A2 must not subtract `bpAll`
+    // for the comparison delta any more.
+    expect(src).not.toMatch(/bp30\s*-\s*bpAll/);
+  });
+
+  it("ships priorMonth + priorYear pcts in the analytics envelope", () => {
+    const ROUTE_SRC = readFileSync(
+      join(ROOT, "src/app/api/analytics/route.ts"),
+      "utf8",
+    );
+    expect(ROUTE_SRC).toContain("bpInTargetPctPriorMonth");
+    expect(ROUTE_SRC).toContain("bpInTargetPctPriorYear");
+  });
+});
+
 describe("v1.4.22 A3 — comparison toggle is global Settings only", () => {
   /**
    * The comparison-overlay toggle is a global preference. Up to v1.4.21

src/app/api/analytics/route.ts

@@ -70,6 +70,16 @@ export const GET = apiHandler(async () => {
   let bpInTargetPct7d: number | null = null;
   let bpInTargetPct30d: number | null = null;
   let bpInTargetPctAllTime: number | null = null;
+  /**
+   * v1.4.22 W5 reconcile (Code-H2) — period-aligned prior-window
+   * pcts so the BD-Zielbereich tile's comparison-overlay caption
+   * stops mismatching its math with its label. The tile's
+   * `compareDelta` is `last30Days - priorMonth` (or `… - priorYear`)
+   * matching the user's `comparisonBaseline` selection, never
+   * `last30Days - allTime` (the v1.4.22 A2 shortcut).
+   */
+  let bpInTargetPctPriorMonth: number | null = null;
+  let bpInTargetPctPriorYear: number | null = null;
   const bpTargets = getBpTargets(user.dateOfBirth);
   if (bpTargets) {
     const now = new Date();
@@ -99,6 +109,8 @@ export const GET = apiHandler(async () => {
     bpInTargetPct7d = windows.last7Days?.pct ?? null;
     bpInTargetPct30d = windows.last30Days?.pct ?? null;
     bpInTargetPctAllTime = windows.allTime?.pct ?? null;
+    bpInTargetPctPriorMonth = windows.priorMonth?.pct ?? null;
+    bpInTargetPctPriorYear = windows.priorYear?.pct ?? null;
   }
 
   // Per-context glucose summaries (canonical mg/dL).
@@ -153,6 +165,8 @@ export const GET = apiHandler(async () => {
     bpInTargetPct7d,
     bpInTargetPct30d,
     bpInTargetPctAllTime,
+    bpInTargetPctPriorMonth,
+    bpInTargetPctPriorYear,
     glucoseByContext,
     correlations,
     healthScore,

src/app/page.tsx

@@ -103,6 +103,15 @@ interface AnalyticsData {
    * on the BD-Zielbereich tile (alongside `7d` and `30d`).
    */
   bpInTargetPctAllTime?: number | null;
+  /**
+   * v1.4.22 W5 reconcile (Code-H2) — period-aligned prior-window
+   * pcts. The BD-Zielbereich tile's comparison-overlay caption picks
+   * `priorMonth` for `comparisonBaseline === "lastMonth"` and
+   * `priorYear` for `lastYear` so the rendered "Δ X% vs. last month"
+   * stays honest. Null when the prior window has no paired readings.
+   */
+  bpInTargetPctPriorMonth?: number | null;
+  bpInTargetPctPriorYear?: number | null;
   glucoseByContext?: Record<string, DataSummaryType>;
 }
 
@@ -817,6 +826,8 @@ export default function DashboardPage() {
           const bp7 = data?.bpInTargetPct7d ?? null;
           const bp30 = data?.bpInTargetPct30d ?? null;
           const bpAll = data?.bpInTargetPctAllTime ?? null;
+          const bpPriorMonth = data?.bpInTargetPctPriorMonth ?? null;
+          const bpPriorYear = data?.bpInTargetPctPriorYear ?? null;
           const bpTrendDelta =
             bp7 !== null && bp30 !== null ? bp7 - bp30 : null;
           const bpSlope30: import("@/lib/analytics/trends").TrendSlope | null =
@@ -835,10 +846,26 @@ export default function DashboardPage() {
                   // mis-interpret a `0` as "very low confidence".
                   confidence: 1,
                 };
+          // v1.4.22 W5 reconcile (Code-H2) — match the comparison
+          // window the user picked. `lastMonth` baseline ⇒ compare
+          // last30 against the now-60d…now-30d window (priorMonth);
+          // `lastYear` baseline ⇒ compare against the matching window
+          // shifted back 365 days. The previous shortcut subtracted
+          // `bpAll` regardless of baseline, which produced a number
+          // whose magnitude was honest but whose label ("vs. last
+          // month") lied to the user.
+          const bpComparePrior =
+            compareBaseline === "lastMonth"
+              ? bpPriorMonth
+              : compareBaseline === "lastYear"
+                ? bpPriorYear
+                : null;
           const bpCompareDelta =
-            compareBaseline === "none" || bp30 === null || bpAll === null
+            compareBaseline === "none" ||
+            bp30 === null ||
+            bpComparePrior === null
               ? null
-              : Math.round((bp30 - bpAll) * 10) / 10;
+              : Math.round((bp30 - bpComparePrior) * 10) / 10;
           trendCards.push({
             id: "bpInTarget",
             order: widgetOrder("bpInTarget"),

src/lib/analytics/__tests__/bp-in-target.test.ts

@@ -476,3 +476,63 @@ describe("computeBpInTargetWindows", () => {
     expect(result.last30Days).toEqual({ pct: 50, pairs: 10 });
   });
 });
+
+describe("computeBpInTargetWindows — period-aligned prior windows (Code-H2)", () => {
+  /**
+   * v1.4.22 W5 reconcile (Code-H2) — the BD-Zielbereich tile's
+   * comparison-overlay caption used to compute `last30Days - allTime`
+   * regardless of the user's `comparisonBaseline` selection. The math
+   * therefore claimed "vs. last month" while subtracting an all-time
+   * average — dishonest by every other tile's standard. The fix is a
+   * pair of period-aligned prior windows shifted back by 30 / 365
+   * days so the tile's `compareDelta` matches its caption.
+   */
+  const NOW = new Date("2026-05-09T12:00:00Z");
+
+  it("returns priorMonth shifted back by 30 days (now-60d…now-30d)", () => {
+    // 30-day window: only one IN paired reading.
+    // Prior-month window (now-60d…now-30d): two paired readings, one IN one OUT.
+    const sys = [
+      reading("2026-05-01T08:00:00Z", 117), // last30 IN
+      reading("2026-04-08T08:00:00Z", 122), // priorMonth IN (32d ago)
+      reading("2026-03-22T08:00:00Z", 145), // priorMonth OUT (48d ago)
+      reading("2026-02-08T08:00:00Z", 128), // older than 60d — neither window
+    ];
+    const dia = [
+      reading("2026-05-01T08:00:00Z", 75),
+      reading("2026-04-08T08:00:00Z", 75),
+      reading("2026-03-22T08:00:00Z", 92),
+      reading("2026-02-08T08:00:00Z", 80),
+    ];
+
+    const result = computeBpInTargetWindows(sys, dia, TARGETS_UNDER_65, NOW);
+    expect(result.last30Days).toEqual({ pct: 100, pairs: 1 });
+    expect(result.priorMonth).toEqual({ pct: 50, pairs: 2 });
+  });
+
+  it("returns priorYear shifted back by 365 days (now-395d…now-365d)", () => {
+    // Pin one paired reading inside the now-395d…now-365d window and one
+    // outside it.
+    const sys = [
+      reading("2025-05-08T08:00:00Z", 117), // 366d ago — INSIDE priorYear, IN
+      reading("2025-04-30T08:00:00Z", 145), // 374d ago — INSIDE priorYear, OUT
+      reading("2025-04-01T08:00:00Z", 117), // 403d ago — OUTSIDE priorYear (too old)
+    ];
+    const dia = [
+      reading("2025-05-08T08:00:00Z", 75),
+      reading("2025-04-30T08:00:00Z", 92),
+      reading("2025-04-01T08:00:00Z", 75),
+    ];
+
+    const result = computeBpInTargetWindows(sys, dia, TARGETS_UNDER_65, NOW);
+    expect(result.priorYear).toEqual({ pct: 50, pairs: 2 });
+  });
+
+  it("returns null for prior windows when no readings fall in them", () => {
+    const sys = [reading("2026-05-01T08:00:00Z", 117)]; // only last-30d
+    const dia = [reading("2026-05-01T08:00:00Z", 75)];
+    const result = computeBpInTargetWindows(sys, dia, TARGETS_UNDER_65, NOW);
+    expect(result.priorMonth).toBeNull();
+    expect(result.priorYear).toBeNull();
+  });
+});

src/lib/analytics/bp-in-target.ts

@@ -221,9 +221,25 @@ export function computeBpInTargetWindows(
   last7Days: { pct: number; pairs: number } | null;
   last30Days: { pct: number; pairs: number } | null;
   allTime: { pct: number; pairs: number } | null;
+  /**
+   * v1.4.22 W5 reconcile (Code-H2) — period-aligned 30-day window
+   * shifted back by one month (now-60d … now-30d). Used by the
+   * BD-Zielbereich tile's comparison-overlay caption when the user's
+   * `comparisonBaseline === "lastMonth"` so the rendered "Δ X% vs.
+   * last month" is honest. Null when the prior window has no pairs.
+   */
+  priorMonth: { pct: number; pairs: number } | null;
+  /**
+   * v1.4.22 W5 reconcile (Code-H2) — same shape, shifted back one
+   * year (now-395d … now-365d) for `comparisonBaseline === "lastYear"`.
+   */
+  priorYear: { pct: number; pairs: number } | null;
 } {
   const sevenDaysAgoMs = now.getTime() - 7 * DAY_MS;
   const thirtyDaysAgoMs = now.getTime() - 30 * DAY_MS;
+  const sixtyDaysAgoMs = now.getTime() - 60 * DAY_MS;
+  const oneYearMinus30DaysAgoMs = now.getTime() - 365 * DAY_MS;
+  const oneYearAgoMs = now.getTime() - 395 * DAY_MS;
 
   const sysLast7 = sysSeries.filter(
     (r) => r.measuredAt.getTime() >= sevenDaysAgoMs,
@@ -238,6 +254,31 @@ export function computeBpInTargetWindows(
     (r) => r.measuredAt.getTime() >= thirtyDaysAgoMs,
   );
 
+  // Period-aligned prior windows. Same 30-day arc, shifted by 30 / 365
+  // days respectively. Bounds are inclusive on the lower end and
+  // exclusive on the upper to avoid double-counting the boundary
+  // reading in both windows.
+  const sysPriorMonth = sysSeries.filter(
+    (r) =>
+      r.measuredAt.getTime() >= sixtyDaysAgoMs &&
+      r.measuredAt.getTime() < thirtyDaysAgoMs,
+  );
+  const diaPriorMonth = diaSeries.filter(
+    (r) =>
+      r.measuredAt.getTime() >= sixtyDaysAgoMs &&
+      r.measuredAt.getTime() < thirtyDaysAgoMs,
+  );
+  const sysPriorYear = sysSeries.filter(
+    (r) =>
+      r.measuredAt.getTime() >= oneYearAgoMs &&
+      r.measuredAt.getTime() < oneYearMinus30DaysAgoMs,
+  );
+  const diaPriorYear = diaSeries.filter(
+    (r) =>
+      r.measuredAt.getTime() >= oneYearAgoMs &&
+      r.measuredAt.getTime() < oneYearMinus30DaysAgoMs,
+  );
+
   return {
     last7Days: computeBpInTargetPct(sysLast7, diaLast7, targets),
     last30Days: computeBpInTargetPct(sysLast30, diaLast30, targets),
@@ -245,5 +286,7 @@ export function computeBpInTargetWindows(
     // analytics route now routes the dashboard tile's headline through
     // this so it stops mirroring `last30Days`.
     allTime: computeBpInTargetPct(sysSeries, diaSeries, targets),
+    priorMonth: computeBpInTargetPct(sysPriorMonth, diaPriorMonth, targets),
+    priorYear: computeBpInTargetPct(sysPriorYear, diaPriorYear, targets),
   };
 }