#1140 Isolate semantic-runtime tests from bridge storage overrides

Test-only, plus one ignored reproduction.

• Cause of the 18-vs-9 failure: views_on_worktrees_and_sessions_embed_each_chunk_once re-embedded every chunk in its second session. Configure gives the process-wide AFT_STORAGE_DIR priority over the request's storage_dir. The bridge tests set that variable under a bridge-only mutex, so a semantic-runtime test could configure its first session into a different storage root. The failure reproduces exactly when storage is switched between sessions.
• Fix: bridge env guards now take the shared process-settings lock and restore the value before releasing it. Semantic-runtime fixtures clear inherited overrides under that lock and assert the resolved storage on every configure. storage_override_holds_shared_process_settings_guard checks from a sibling thread that the lock is held. It failed before the fix.
• Separate gap, not fixed here: views_on_two_processes_reproduce_duplicate_checkout_chunk (ignored) shows that two standalone aft processes sharing one storage both embed an identical checkout-own chunk, because file claims and the batch gate are in-process only. It needs a reviewed cross-process design before views can become the default.

Verified: per_checkout_semantic_runtime plus the guard test, twice on Linux (9 passed, 1 ignored), the formatting check and the Windows target check.

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Summary by cubic

Fixes a test isolation bug where bridge tests' AFT_STORAGE_DIR override leaked into semantic-runtime tests, causing a session to re-embed all chunks (18 instead of 9).

• Bridge env guards now hold the shared process-settings lock and restore the env var before releasing it; they skip acquisition while a bridge runner owns the lock to avoid deadlocking the runner. Feature-gated bridge env helpers now use this same setter.
• Semantic-runtime fixtures clear inherited overrides under the same lock and assert resolved storage on every configure.
• Adds an ignored reproduction showing two standalone aft processes sharing one storage both embed an identical chunk; a cross-process claim design needs review before fixing.