Run the full test suite against both temporal-polyfill and @js-temporal/polyfill

[dogmar]

Motivation

The library no longer bundles a Temporal implementation — callers must provide one (see the temporal prop docs), and we recommend two polyfills:

• temporal-polyfill (fullcalendar)
• @js-temporal/polyfill (js-temporal)

These are independent implementations and can differ in edge-case behavior (parsing, toLocaleString/calendar handling, overflow, DST). To be confident the library works on both, every test should run against both — not just the reference implementation.

Current state

• package/src/test-temporal.ts exports a temporalVariants matrix, but after the mini-shim removal it has a single entry (@js-temporal/polyfill).
• Only a few suites consume the matrix via describe.each(temporalVariants): utils.test.ts, keyboard.test.ts.
• ~10 suites hardcode @js-temporal/polyfill directly instead of using the matrix: calendar-provider.test.tsx, month-view.test.tsx, weeks-view.test.tsx, overflow.test.ts, compute-weeks-in-window.test.ts, resolve-first-week.test.ts, weeks-keyboard.test.ts, grid.perf.test.tsx, grid.event-perf.test.tsx.
• temporal-polyfill (fullcalendar) is not a devDependency yet (only @js-temporal/polyfill is).

Proposed work

• Add temporal-polyfill (fullcalendar) as a devDependency (via the pnpm catalog).
• Add it as a second entry in temporalVariants (package/src/test-temporal.ts), shaped as a TemporalNamespace like the existing entry.
• Migrate every hardcoded suite to source its Temporal from temporalVariants and wrap its cases in describe.each(temporalVariants)("… ($name)", ({ T }) => { … }), so each test executes once per polyfill. Replace the local temporal/Temporal constants accordingly.
• Decide and document how perf suites (grid.perf.test.tsx, grid.event-perf.test.tsx) participate — run on both, or pin to one with a noted rationale (matrixing perf benchmarks may just double runtime without added signal).
• Make describe.each(temporalVariants) the default pattern for new date-dependent tests (note in CONTRIBUTING or a comment in test-temporal.ts).

Acceptance criteria

• vp run -r test executes the full date-dependent suite once per polyfill, and passes on both.
• Adding a third implementation later means only adding one entry to temporalVariants.
• No test file imports a polyfill directly except test-temporal.ts (and any intentionally-pinned perf suite).

Notes

Some assertions are format-sensitive (e.g. PlainYearMonth.toString(), toLocaleString output). Where the two polyfills format differently, prefer semantic assertions (compare via Temporal.X.compare, check fields) over exact-string matches, or branch on the variant where unavoidable.