feat(lint): refuse UTC day keys outside the time zone module

`instant.toISOString().slice(0, 10)` and parsing a key back as UTC
midnight are the idioms behind every day-boundary fix this year: the
evening west of UTC and the small hours east of it land on the
neighbouring day. A new rule, healthlog/no-utc-day-key, refuses them in
src/ outside src/lib/tz and tests, and refuses a disable directive for
itself that is not next-line or gives no reason.

The time zone module now names every legitimate form: userDayKey for a
person's day, dateOnlyKey / dateOnlyAtNoonUtc / dayKeyAsUtcMidnight for
date-only values and day labels, daysBetweenDateKeys / weekdayOfDateKey /
isCalendarDateKey / statedDateKey for key arithmetic and parsing.

Of the 133 sites the rule found, 84 are gone. Most moved onto a named
helper with no change in behaviour; these changed behaviour:

- cycle calendar, insights, Coach snapshot and prediction refresh start
  their lookbacks at the first instant of the user's day
- a model-transcribed date like "June 10, 2026" no longer lands a day
  early in a process east of UTC (documents and lab scans)
- a custom report range covers exactly the picked days in the profile
  zone instead of starting the evening before west of UTC
- date-of-birth pickers cap at the user's today, not the UTC one
- an allergy onset date is stored at noon UTC like other date-only values
- the report's cycle summary, the Coach's usual BP range and the intake
  projector's rollup recompute use the user's day
- vaccination dates render as dates instead of the previous evening

49 remain, each with its reason on the line above: 39 UTC by design (15
file name stamps, rollup and object keys, the UTC-day score engines) and
10 marked baseline. A guard test requires a classified reason on every
directive and holds the baseline to a ceiling that only goes down.

src/app/api/cycle/calendar/route.ts

@@ -43,6 +43,7 @@ import { moodDateKey } from "@/lib/mood/date-key";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { getServerTranslator } from "@/lib/i18n/server-translator";
 import { resolveServerLocale } from "@/lib/i18n/server-locale";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 
 const DEFAULT_PAST_DAYS = 90;
 const DEFAULT_FORWARD_DAYS = 180;
@@ -148,7 +149,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
         deletedAt: null,
         type: "WRIST_TEMPERATURE",
         measuredAt: {
-          gte: new Date(Date.parse(`${addDays(today, -90)}T00:00:00Z`)),
+          gte: startOfLocalDayKey(addDays(today, -90), tz),
         },
       },
       orderBy: { measuredAt: "asc" },

src/app/api/cycle/insights/route.ts

@@ -45,6 +45,7 @@ import {
 import { addDays } from "@/lib/cycle/day-math";
 import { moodDateKey } from "@/lib/mood/date-key";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 
 /** Trailing window the phase-contrast walks (days). */
 const WINDOW_DAYS = 365;
@@ -124,7 +125,7 @@ export const GET = apiHandler(async () => {
         deletedAt: null,
         type: "WRIST_TEMPERATURE",
         measuredAt: {
-          gte: new Date(Date.parse(`${addDays(today, -90)}T00:00:00Z`)),
+          gte: startOfLocalDayKey(addDays(today, -90), tz),
         },
       },
       orderBy: { measuredAt: "asc" },
@@ -140,7 +141,7 @@ export const GET = apiHandler(async () => {
             deletedAt: null,
             type: { in: PHASE_CROSSTAB_METRIC_TYPES },
             measuredAt: {
-              gte: new Date(Date.parse(`${from}T00:00:00Z`)),
+              gte: startOfLocalDayKey(from, tz),
             },
           },
           orderBy: { measuredAt: "asc" },
@@ -166,7 +167,7 @@ export const GET = apiHandler(async () => {
           where: {
             userId: user.id,
             deletedAt: null,
-            moodLoggedAt: { gte: new Date(Date.parse(`${from}T00:00:00Z`)) },
+            moodLoggedAt: { gte: startOfLocalDayKey(from, tz) },
           },
           orderBy: { moodLoggedAt: "asc" },
           select: { score: true, moodLoggedAt: true },