perf(coach): read only coarse bands for the deep-history tail

Add a coarseOnly option to buildTieredSeries that skips the raw 0–14d
read and the DAY (14–30d) / WEEK (30–90d) band reads, returning only the
MONTH / YEAR bands plus the anomaly envelope. The Coach snapshot's
coarse timeline tail consumes only month/year + anomalies, so the
near-term bands were read and discarded every turn; it now passes
coarseOnly. The briefing path is unchanged. Look the learn-guide prompt
example up by slug rather than by array index so reordering the catalog
cannot silently change the example link.

src/lib/ai/coach/learn-catalog.ts

@@ -100,11 +100,16 @@ export const LEARN_GUIDES: readonly LearnGuide[] = [
  */
 export function learnCatalogPromptBlock(): string {
   const lines = LEARN_GUIDES.map((g) => `- ${g.topic}: ${g.url}`).join("\n");
+  // Look the example guide up by slug, not by array index, so reordering the
+  // catalog above cannot silently change the prompt example.
+  const exampleGuide =
+    LEARN_GUIDES.find((g) => g.slug === "resting-heart-rate") ??
+    LEARN_GUIDES[0];
   return [
     "LEARN GUIDES (public articles you MAY link when genuinely helpful):",
     lines,
     "Only link a URL from this list — never invent a /learn URL or article",
     "content. Reference at most one guide, and only when it adds real value,",
-    'e.g. "more on this: ' + LEARN_GUIDES[1].url + '".',
+    'e.g. "more on this: ' + exampleGuide.url + '".',
   ].join("\n");
 }

src/lib/ai/coach/snapshot.ts

@@ -408,12 +408,13 @@ async function buildCoarseTimelineTail(
   // "fall back on miss" contract the rest of the snapshot honours).
   let series: Awaited<ReturnType<typeof buildTieredSeries>>;
   try {
-    // `skipRecentDaily` — the snapshot already holds the raw 0–14d rows from
-    // its own window read, so the tiered builder skips that duplicate raw
-    // round-trip and returns only the coarse MONTH/YEAR bands + anomalies.
+    // `coarseOnly` — the snapshot already holds the raw 0–14d rows from its
+    // own window read and this tail consumes only the MONTH/YEAR bands +
+    // anomaly envelope, so the builder skips the raw read AND the DAY/WEEK
+    // band reads it would otherwise discard.
     series = await buildTieredSeries(userId, type, {
       now: now.getTime(),
-      skipRecentDaily: true,
+      coarseOnly: true,
     });
   } catch {
     return undefined;

src/lib/rollups/tiered-context.ts

@@ -124,6 +124,16 @@ export interface BuildTieredSeriesOptions {
    * MONTH/YEAR bands + anomaly envelope; avoids a duplicate raw round-trip.
    */
   skipRecentDaily?: boolean;
+  /**
+   * v1.18.7 — skip the raw 0–14d read AND the DAY (14–30d) / WEEK (30–90d)
+   * band reads, returning only the MONTH / YEAR bands plus the anomaly
+   * envelope. For the Coach's `buildCoarseTimelineTail`, which consumes only
+   * month/year + anomalies; the skipped near-term bands would otherwise be
+   * read and discarded. The anomaly window-stats then span the coarse bands
+   * only, which is the right baseline for the deep-history tail. Implies
+   * `skipRecentDaily`.
+   */
+  coarseOnly?: boolean;
 }
 
 interface RollupBucket {
@@ -280,13 +290,19 @@ export async function buildTieredSeries(
     await ensureUserRollupsFresh(userId);
   }
 
+  const coarseOnly = options.coarseOnly ?? false;
+  const emptyBand = Promise.resolve<RollupBucket[]>([]);
   const [recentDaily, dayBand, weekBand, monthBand, yearBand] =
     await Promise.all([
-      options.skipRecentDaily
+      options.skipRecentDaily || coarseOnly
         ? Promise.resolve<TieredRawPoint[]>([])
         : readRecentDaily(userId, type, now),
-      readBand(userId, type, "DAY", TIERED_BANDS.dayUntil, TIERED_BANDS.rawDays, now),
-      readBand(userId, type, "WEEK", TIERED_BANDS.weekUntil, TIERED_BANDS.dayUntil, now),
+      coarseOnly
+        ? emptyBand
+        : readBand(userId, type, "DAY", TIERED_BANDS.dayUntil, TIERED_BANDS.rawDays, now),
+      coarseOnly
+        ? emptyBand
+        : readBand(userId, type, "WEEK", TIERED_BANDS.weekUntil, TIERED_BANDS.dayUntil, now),
       readBand(userId, type, "MONTH", TIERED_BANDS.monthUntil, TIERED_BANDS.weekUntil, now),
       readBand(userId, type, "YEAR", TIERED_BANDS.yearUntil, TIERED_BANDS.monthUntil, now),
     ]);