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/export/records-backup.ts

@@ -32,6 +32,7 @@ import {
   type AllergyDTO,
   type FamilyHistoryEntryDTO,
 } from "@/lib/records/dto";
+import { dateOnlyKey } from "@/lib/tz/date-only";
 
 /** Disclosed in the payload manifest AND mirrored in the export UI copy. */
 export const DOCUMENTS_MANIFEST_NOTE =
@@ -543,10 +544,8 @@ export async function buildRecordsBackupSection(
       mimeType: d.mimeType,
       byteSize: d.byteSize,
       status: d.status,
-      reportDate: d.reportDate ? d.reportDate.toISOString().slice(0, 10) : null,
-      documentDate: d.documentDate
-        ? d.documentDate.toISOString().slice(0, 10)
-        : null,
+      reportDate: d.reportDate ? dateOnlyKey(d.reportDate) : null,
+      documentDate: d.documentDate ? dateOnlyKey(d.documentDate) : null,
       sourceSystem: d.sourceSystem,
       sourceId: d.sourceId,
       sourceInstance: d.sourceInstance,