Extracted _post_init_callback_result(app) -> Result[None] helper above
the App._post_init method.
ANTI-SLIMING: full Result[T] propagation (NO pass-after-logging). The
helper returns Result(data=None) on success or Result(data=None,
errors=[ErrorInfo]) on exception (logging NOT a drain per the user's
principle 2026-06-17).
The legacy _post_init method preserves its signature and calls the helper,
draining errors to self._startup_timeline_errors.
Tests: 2 new tests verify both paths (success and RuntimeError).
Audit: L612 reclassified from INTERNAL_SILENT_SWALLOW (10 sites remaining,
was 11). New helper L612 is INTERNAL_COMPLIANT.
ed
e761244c4a