fix(calendar): honor the Unix epoch as a reference time

[OskarEichler]

Fix

Honor numeric Unix timestamp 0 as the calendar reference. It was previously treated as an omitted reference and replaced with the current date.

dayjs.extend(calendar)
dayjs(0).calendar(0)
// UTC before: '01/01/1970'; after: 'Today at 12:00 AM'
// The result now matches dayjs(0).calendar(new Date(0)).

Compatibility

No breaking API, dependency, or runtime changes. Only the incorrect numeric-zero reference changes. Existing omitted/null/falsy fallback behavior is preserved, and calendar callbacks still receive the current time as documented—not the supplied reference. Formatting, locale selection, and category thresholds are unchanged.

Verification

• 50 focused checks for epoch/near-epoch references across calendar categories, omitted/null/falsy references, callback scope, and callback time.
• All 794 existing tests / 93 suites and 100% line coverage on the reviewed patch set, including this change and independent locale/minMax fixes.
• Lint, all 40 declaration files, complete package builds, and size limit pass; the release 1.11.23 backport also passes its existing suite/builds.

No tests were added or modified. The legacy size checker required a process-local OpenSSL legacy-provider flag on Node 22; no package configuration change was made.