fix(core): keep resource date round trips timezone-stable (#2431)

[MiddleSokilAI]

Repro

With PHP date.timezone=Asia/Tokyo, a resource date entered as 2026/09/01 10:00:00 is displayed again as 2026/09/01 01:00:00 after save/reopen. Saving again compounds the offset.

Root Cause

toDateFormat() formats stored timestamps in UTC through Carbon::createFromTimestamp($timestamp, 'UTC'), while toTimeStamp() parsed manager date fields through mktime(), which uses PHP's runtime timezone. Non-UTC environments therefore shifted the timestamp on every manager round trip.

Change Summary

• Parse manager date strings with gmmktime() so toTimeStamp() uses the same UTC basis as toDateFormat().
• Add a regression test covering an Asia/Tokyo open/save/open round trip.

Validation

• git diff --check
• wsl php -l core/src/Core.php
• wsl php -l core/tests/Unit/CoreTest.php
• WSL smoke test: Asia/Tokyo 2026/09/01 10:00:00 round-trips through toTimeStamp() and toDateFormat() without changing on the second conversion.

vendor/bin/pest is not available in the current local core/vendor, so the full Pest suite was not run locally.

Risk

Low. The fix is scoped to the shared date parser and aligns it with the existing UTC display path. It affects manager date strings that already round-trip through toDateFormat() / toTimeStamp().