refactor(tz): route day-key copies through the canonical userDayKey

Compliance streaks, the measurement-consolidation day key, and the
correlations weekday helper each carried their own Intl formatter
(en-CA / sv-SE / short-weekday). Delegate them to userDayKey and
wallClockInTz so every day-bucket key stays byte-for-byte aligned and the
locale decision lives in one place.

src/lib/analytics/correlations-fast-path.ts

@@ -58,6 +58,7 @@ import {
   type CorrelationResult,
 } from "@/lib/insights/correlations";
 import { isNearUtc, userDayKey } from "@/lib/tz/resolver";
+import { wallClockInTz } from "@/lib/tz/wall-clock";
 
 /**
  * v1.4.37 W2 — cold-path correlation window. Trim from 30 to 28 days
@@ -368,34 +369,8 @@ function dateFromDayKey(key: string): Date {
   return new Date(`${key}T00:00:00.000Z`);
 }
 
-const WEEKDAY_FORMATTER_CACHE = new Map<string, Intl.DateTimeFormat>();
-function getWeekdayFormatter(timeZone: string): Intl.DateTimeFormat {
-  let formatter = WEEKDAY_FORMATTER_CACHE.get(timeZone);
-  if (!formatter) {
-    formatter = new Intl.DateTimeFormat("en-CA", {
-      timeZone,
-      year: "numeric",
-      month: "2-digit",
-      day: "2-digit",
-      weekday: "short",
-    });
-    WEEKDAY_FORMATTER_CACHE.set(timeZone, formatter);
-  }
-  return formatter;
-}
-
-const ISO_WEEKDAY: Record<string, number> = {
-  Mon: 1,
-  Tue: 2,
-  Wed: 3,
-  Thu: 4,
-  Fri: 5,
-  Sat: 6,
-  Sun: 7,
-};
-
 function isoWeekdayInTz(d: Date, timeZone: string): number {
-  const parts = getWeekdayFormatter(timeZone).formatToParts(d);
-  const weekday = parts.find((p) => p.type === "weekday")?.value ?? "Mon";
-  return ISO_WEEKDAY[weekday] ?? 1;
+  // `wallClockInTz` returns 0=Sun..6=Sat; remap to ISO 1=Mon..7=Sun.
+  const weekday = wallClockInTz(d, timeZone).weekday;
+  return weekday === 0 ? 7 : weekday;
 }

src/lib/measurements/consolidation-tz.ts

@@ -22,6 +22,9 @@
  * every existing import site keeps working unchanged.
  */
 import type { MeasurementType } from "@/generated/prisma/client";
+// `userDayKey` lives in the pure `@/lib/tz/format` leaf (no Prisma /
+// node:module pull) so importing it keeps this dependency-free module clean.
+import { userDayKey } from "@/lib/tz/format";
 
 /**
  * Canonical grace window shared by the cumulative + mean drains. Rows
@@ -52,12 +55,12 @@ export interface PerSampleRow {
 
 /**
  * Resolve the user's calendar-day key (`YYYY-MM-DD`) for a given
- * timestamp + timezone. Reuses the same `sv-SE` Intl formatting choice
- * the mood-entries path locked in v1.4.25 W7b so iOS-side and
- * server-side day-keys round-trip byte-identically.
+ * timestamp + timezone. Delegates to the canonical `userDayKey` so the
+ * server-side and iOS-side day-keys round-trip byte-identically against
+ * every other day-bucketed surface.
  */
 export function dayKeyForUserTz(date: Date, tz: string): string {
-  return new Intl.DateTimeFormat("sv-SE", { timeZone: tz }).format(date);
+  return userDayKey(date, tz);
 }
 
 /**

src/lib/medications/scheduling/compliance.ts

@@ -39,6 +39,7 @@ import {
   type RecurrenceContext,
 } from "./recurrence";
 import { normaliseDoseWindows } from "./worker-helpers";
+import { userDayKey } from "@/lib/tz/resolver";
 
 export interface ComplianceChips {
   /** 0-100, taken / (taken + missed). Skipped doses are excluded from
@@ -62,9 +63,10 @@ const DAY_MS = 24 * 60 * 60 * 1000;
 
 /**
  * Day-key in the user's timezone (or system-local when `tz` is
- * undefined). `Intl.DateTimeFormat` with `en-CA` reliably returns
- * `YYYY-MM-DD`, which sorts lexically and matches the chart
- * legend's `day` ticks.
+ * undefined). Routes through the canonical `userDayKey` so the
+ * `YYYY-MM-DD` bucket stays byte-for-byte aligned with every other
+ * day-bucketed surface; the host-local branch covers the pre-per-user
+ * callers that pass no zone.
  */
 function localDayKey(d: Date, tz: string | undefined): string {
   if (!tz) {
@@ -73,12 +75,7 @@ function localDayKey(d: Date, tz: string | undefined): string {
     const day = String(d.getDate()).padStart(2, "0");
     return `${y}-${m}-${day}`;
   }
-  return new Intl.DateTimeFormat("en-CA", {
-    timeZone: tz,
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-  }).format(d);
+  return userDayKey(d, tz);
 }
 
 /**