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/lib/rollups/mood-rollups.ts

@@ -55,6 +55,7 @@ import type {
   PrismaClient,
   RollupGranularity,
 } from "@/generated/prisma/client";
+import { dayKeyAsUtcMidnight } from "@/lib/tz/date-only";
 
 /** Granularities that recompute asynchronously via pg-boss. */
 const ASYNC_GRANULARITIES: RollupGranularity[] = ["WEEK", "MONTH", "YEAR"];
@@ -250,7 +251,7 @@ export async function recomputeMoodBucketsForEntry(
   // on `entry.date`) and puts a late-evening / after-midnight local
   // mood on the calendar day the user logged it in their own timezone.
   // `bucket_start` encodes the label as its UTC midnight instant.
-  const bucketStart = new Date(`${dateLabel}T00:00:00.000Z`);
+  const bucketStart = dayKeyAsUtcMidnight(dateLabel);
   await client.$executeRaw`
     WITH aggregate AS (
       SELECT
@@ -718,7 +719,7 @@ function bucketSpan(
 ): { from: Date; to: Date } {
   // Anchor at the label's UTC midnight; its UTC calendar parts ARE the
   // label's parts, so the existing UTC date math below is exact.
-  const anchor = new Date(`${dateLabel}T00:00:00.000Z`);
+  const anchor = dayKeyAsUtcMidnight(dateLabel);
   switch (granularity) {
     case "DAY": {
       return {