Premium Analytics: converge the duplicated date-label validators and day bounds

[chihsuan]

Fixes WOOA7S-2079 (items 3, 4, 7 and part of 5; item 1 is #52064)

Proposed changes

• Extract one parseExactLabel( label, labelFormat ) into packages/datetime and route the five copies of the parse + isValid + round-trip format idiom through it, along with the two copies of its new Date( 2001, 0, 1 ) reference date.
• Delete toValidDay in use-post-views.ts, which duplicated the shared toDay character for character. Its sibling widget already imported the shared one.
• Name the '00:00:00' / '23:59:59' day bounds as DAY_START_TIME / DAY_END_TIME in stats/utils.ts, next to the three files that use them.

No behavior changes, so nothing in readme.txt and no plugin changelog entries. The only test file edited gains assertions; none was moved or weakened.

Related product discussion/links

• WOOA7S-2079

Does this pull request change what data or activity we track or use?

No.

Testing instructions

Nothing to click. This is an internal refactor with no user-visible surface, so the check is that the existing suites still pass without any assertion being edited.

• jp test js packages/premium-analytics (or, scoped: PA_TEST_GROUPS=1 TZ=UTC npx jest --config=tests/jest.config.cjs packages/datetime packages/data widgets/post-views widgets/popular-days packages/widgets-toolkit), 166 suites, 1531 tests.
• pnpm run typecheck and pnpm run lint-changed.
• Reviewing the diff, the thing worth checking is that each rewritten call site is equivalent, in particular the Date | null to undefined conversion in readRowDate and the reference-date consolidation.

[github-actions]

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[github-actions]

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Code Coverage Summary

Coverage changed in 6 files. Only the first 5 are listed here.

File	Coverage	Δ%	Δ Uncovered
projects/packages/premium-analytics/packages/data/src/processing/stats/post.ts	24/24 (100.00%)	0.00%	0 💚
projects/packages/premium-analytics/packages/data/src/processing/stats/time-series.ts	100/105 (95.24%)	-0.04%	0 💚
projects/packages/premium-analytics/packages/data/src/processing/stats/utils.ts	102/104 (98.08%)	0.04%	0 💚
projects/packages/premium-analytics/packages/datetime/src/date.ts	23/23 (100.00%)	0.00%	0 💚
projects/packages/premium-analytics/widgets/popular-days/bucket-views-by-weekday.ts	18/18 (100.00%)	4.55%	-1 💚

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