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/managed-profiles/route.ts

@@ -17,6 +17,7 @@ import { toManagedProfileView } from "@/lib/managed-profiles/lifecycle";
 import { annotate } from "@/lib/logging/context";
 import { checkRateLimit } from "@/lib/rate-limit";
 import { createManagedProfileSchema } from "@/lib/validations/managed-profiles";
+import { dayKeyAsUtcMidnight } from "@/lib/tz/date-only";
 
 /**
  * The same ceiling its guardian sibling carries, and for the same reason.
@@ -59,7 +60,7 @@ export const POST = apiHandler(async (request: NextRequest) => {
     creatorId: user.id,
     displayName: parsed.data.displayName,
     dateOfBirth: parsed.data.dateOfBirth
-      ? new Date(`${parsed.data.dateOfBirth}T00:00:00.000Z`)
+      ? dayKeyAsUtcMidnight(parsed.data.dateOfBirth)
       : null,
     locale: parsed.data.locale,
     timezone: parsed.data.timezone,