parser: Add IANA time_zone support for native timestamps [Backport to 4.2]

[cosmo0920]

Backporting of #11913 except for integration tests.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Honor TZif POSIX tails for future DST

When the host ships a TZif v2/v3/v4 file that relies on the POSIX footer for transitions beyond the last explicit record (common with slim tzdata), this path parses only the second transition block and discards the footer. After that last explicit transition, tzif_type_at_utc() will keep using the final offset forever, so a parser configured with e.g. time_zone America/New_York can convert future summer timestamps after the file's explicit transition range one hour late. Please parse the TZ string footer or delegate to the platform timezone logic for dates beyond the final transition.

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