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/components/cycle/bbt-chart.tsx

@@ -44,6 +44,7 @@ import { prefersReducedMotion } from "@/lib/charts/reduced-motion";
 import { cn } from "@/lib/utils";
 import type { CalendarDay, CervicalMucus, OvulationTest } from "./types";
 import { PHASE_HUE, OVULATION_HUE } from "./phase-tokens";
+import { shiftDateKey } from "@/lib/tz/format";
 
 /** Trailing fallback window (days) when no MENSTRUAL-anchored cycle is active. */
 const FALLBACK_WINDOW_DAYS = 35;
@@ -97,9 +98,7 @@ export function BbtChart({
   // recent temperatures.
   const fromDate = useMemo(() => {
     if (cycleStartDate) return cycleStartDate;
-    const fallback = new Date(ymdToMs(today));
-    fallback.setUTCDate(fallback.getUTCDate() - FALLBACK_WINDOW_DAYS);
-    return fallback.toISOString().slice(0, 10);
+    return shiftDateKey(today, -FALLBACK_WINDOW_DAYS);
   }, [cycleStartDate, today]);
 
   const points = useMemo<BbtPoint[]>(() => {