fix(tz): one start-of-day for every zone, including skipped midnights

Local day bounds had three implementations. The day-key one read the zone
offset at UTC midnight, which on a transition day in zones far from UTC
(Pacific/Auckland, Australia/Sydney) is the offset after the change, so the
day started an hour off. The instant one converged on local 00:00, which in
zones that skip midnight (America/Santiago, America/Havana) does not exist,
and returned 23:00 of the previous day.

startOfLocalDayKey and localDayWindow in lib/tz/local-day are now the only
implementation: the first instant that reads the day, tried under every
offset in force around it. A skipped wall clock resolves to the moment the
clocks jump, the day-keyed copies in the measurement drains, the compliance
rollups and the dashboard batch window are gone, and the noon and mid-hour
anchors use the same converge.

src/app/api/measurements/route.ts

@@ -34,8 +34,8 @@ import { pickCanonicalSourceRows } from "@/lib/analytics/source-priority";
 import {
   canonicalDailyTimestamp,
   dayKeyForUserTz,
-  localDayWindow,
 } from "@/lib/measurements/drain-per-sample-cumulative";
+import { localDayWindow } from "@/lib/tz/local-day";
 import { withIdempotency } from "@/lib/idempotency";
 import { isP2002 } from "@/lib/prisma-errors";
 import { encryptNote, shapeMeasurementNotes } from "@/lib/crypto/note-cipher";