fix: normalize release archive timestamps in UTC

[codeforester]

Summary

Normalize staged file and directory mtimes in UTC so the same commit produces byte-identical release assets across host timezones. Document deterministic locale ordering and the supported GNU tar/bsdtar boundary.

Issue

Fixes #511

Validation

• bats --filter "release artifact bytes are independent of the host timezone" tests/release-artifact.bats — passed (1 test); builds under TZ=UTC and TZ=Asia/Kolkata and compares the complete output sets.
• git diff --check — passed.
• Full repository hosted validation is pending.

Demo Impact

None.

Docs Impact

Document UTC timestamp normalization, C-locale ordering, and supported tar implementations.

API Impact

None.

CI Impact

Adds a cross-timezone full-artifact reproducibility test.

Security Notes

None.

[codeforester]

Confirmed fixed. Now uses local -x TZ=UTC0 scoped once at function level (matching lib_std.sh's established convention), inherited by both loops instead of duplicated. Independently rebuilt the archive under TZ=UTC0 and TZ=Asia/Kolkata and diffed the bytes — identical. 12/12 tests pass.