Resolve macOS timezone names relative to zoneinfo

[OskarEichler]

Summary

Extract the IANA name after /zoneinfo/ instead of assuming a fixed number of path components in /etc/localtime's symlink target.

On the inspected macOS installation, the target is /var/db/timezone/zoneinfo/Europe/Istanbul. The current code returns "zoneinfo/Europe/Istanbul", which TZInfo rejects; local detection then falls back to an unrelated zone with matching current offsets.

Reproduction

Before: EtOrbi.osx_tz returns "zoneinfo/Europe/Istanbul"; automatic detection selected Antarctica/Syowa. After: it returns Europe/Istanbul and detection selects that zone.

Checked the actual symlink and temporary readlink substitutions for /usr/share/zoneinfo/Asia/Tokyo and /var/db/timezone/zoneinfo/America/New_York. The traditional path remains supported; a target without a zoneinfo component yields nil for existing fallback handling.

Verification

• Ruby 4.0.6 via rbenv; existing Probatio suite, no test files added or modified.
• Reviewed master baseline and this individual patch each run 270 tests / 412 assertions with exactly the same three pre-existing DST/rday failures from rday / rweek calculation is odd in time zones with DST #51. Failure identities were compared, not just counts.
• A release-based 1.4.1 branch containing the five separately proposed fixes passes 263 existing tests / 405 assertions with TZInfo 2.0.6 in UTC, Asia/Tokyo, America/Chicago, Europe/Istanbul and the detected local zone.
• Temporary focused before/after reproductions pass with TZInfo 2.0.6 and 1.2.11. The combined release branch also passes the existing TZInfo 1.2.11 suite at seed 36231. Seed 99751 has the same pre-existing Zulu test-order failure on both unmodified release and patched release: a test mutates the cached TZInfo object's comparison method.
• Ruby syntax and gem packaging checks pass. Targeted Lint passes on the combined branch with the existing, unrelated AssignmentInCondition warning excluded; no lint configuration was changed.

The current master's unreleased day-delta change was not brought into the consumer branch. No live scheduler or production operations were used. Linux/Windows and other Ruby runtimes were not run locally.

Breaking changes and limitations

No intended breaking API change. Local detection now retains the actual named timezone instead of a fallback with potentially different historical/DST rules. Explicit ENV['TZ'] and Rails Time.zone priority are unchanged. No operating-system files or settings were modified. No overlapping current fix was found.