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/medications/[id]/intake/[eventId]/route.ts

@@ -28,6 +28,7 @@ import {
   resolveSlotForWriteByBand,
 } from "@/lib/medications/scheduling/slot-upsert";
 import { NextRequest } from "next/server";
+import { dateOnlyKey } from "@/lib/tz/date-only";
 
 type RouteParams = { params: Promise<{ id: string; eventId: string }> };
 
@@ -100,7 +101,7 @@ export const PUT = apiHandler(
       });
       if (medication?.startsOn) {
         const takenDay = dayKeyForUserTz(data.takenAt, user.timezone);
-        const startsOnDay = medication.startsOn.toISOString().slice(0, 10);
+        const startsOnDay = dateOnlyKey(medication.startsOn);
         if (takenDay < startsOnDay) {
           annotate({
             action: {