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/cycle/prediction-refresh.ts

@@ -27,6 +27,7 @@ import { addDays } from "@/lib/cycle/day-math";
 import { BBT_WINDOW } from "@/lib/cycle/types";
 import { moodDateKey } from "@/lib/mood/date-key";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
 
 /** Day-log lookback: the calendar's default past span or the symptothermal
  *  window, whichever reaches further back. */
@@ -97,10 +98,9 @@ export async function refreshPredictionCacheForUser(
         deletedAt: null,
         type: "WRIST_TEMPERATURE",
         measuredAt: {
-          gte: new Date(
-            Date.parse(
-              `${addDays(today, -TEMPERATURE_LOOKBACK_DAYS)}T00:00:00Z`,
-            ),
+          gte: startOfLocalDayKey(
+            addDays(today, -TEMPERATURE_LOOKBACK_DAYS),
+            tz,
           ),
         },
       },