test(ci): isolate semantic probe fixture coverage - #5375

The remaining typescript-core (1/3) failure is confirmed on main run 36770551543 and comes from the duplicate bare SHA-256 matcher introduced by #5364, not from this PR. A minimal main-based fix is now in #5377; this branch remains unchanged at exact head 086f2643355d36b62ccf74cbdac6200ca31a498f.

A second inherited failure appeared in test-shard (2): test_runtime_source_churn_has_a_stable_readiness_diagnostic observed ready instead of package_invalid. The current main fixture deletes later.ts from one parallel read without synchronizing the other read; if later.ts is already prefetched, the expected missing-file retry does not occur. #5367 makes this race deterministic with an Event and its exact-head shard 2 passed. This is independent of the sole coverage-helper change in #5375, so no code change is planned here.

The newly completed test-shard (4) failure is the second assertion covered by #5367: test_runtime_request_source_churn_raises_a_stable_startup_diagnostic observed runtime_exited_before_ready instead of packaged_runtime_source_unstable. #5367 synchronizes this source-churn fixture and its exact-head test-shard (4) passed. This is independent of #5375’s sole coverage-helper change, so this branch remains unchanged.

The final test-shard (3) failure is another inherited main regression: #5366 advanced the usage notice to version 5 but left test_source_first_usage_disclosure_keeps_json_pure_and_does_not_send pinned to 4. I reproduced the exact 5 == 4 assertion on clean origin/main@9c9b90d8e; the one-line test correction is isolated in #5379. No #5375 code change is needed.

Exact-head update for 254d66b29341382c4d4c3b94382806e0bb59ae93:

• Synced with repaired main@638d0e39cff09948df1816a4a5c5ccce70262c57 through a conflict-free signed-off merge commit.
• The PR-owned diff remains limited to 9 added test lines in tests/architecture/test_semantic_development_probe.py.
• Focused Python validation passed: 74/74 across the semantic probe, source CLI entrypoint, and Turn journal runtime-readiness suites.
• npm run typecheck:control-plane passed.
• tests/control_plane_ts/content_digest_single_owner.test.ts passed: 17/17.
• examples/control_plane/cli-output-budget-regression-smoke.py passed against current origin/main.
• git diff --check passed.

The previous run was based on main@64402b4a8, whose duplicate BARE_SHA256_PATTERN import made the TypeScript runtime exit before ready. Shards 1, 2, and 4 failed from that unavailable runtime, and shard 3 was cancelled after the run was already red. #5391 repaired and landed that main regression.

Fresh exact-head CI run 36818756640 is complete: 24 jobs passed, 2 optional jobs were skipped, and no jobs failed. No #5375-owned failure was found, and no merge action was taken.

Exact-head update for aa1ef4299827a1bd01fc508150bd38704e90d269:

• Synced through signed-off, conflict-free merge commits with current main@3ad3269af4d2cfae2085693823dcfcda522f9a2b.
• Focused validation passed: 74 Python tests, 17 digest-owner TypeScript tests, control-plane TypeScript typecheck, Ruff, and git diff --check.
• After test(host): publish process markers atomically #5365 entered main during validation, the affected Host/Codex CLI suite also passed: 48/48.

Exact-head CI run 36825867638 is complete: 24 jobs passed, 2 optional jobs were skipped, and no jobs failed. This includes all four Python shards, combined coverage, and merge-gate. The PR is ready for maintainer review; no merge action was taken.

Exact-head update for 3d2e9862bb93aa27fc9c349c47ab03b2c5dbc923 on main@060e4698a39264988391b3f11bc18c91eb81d27f:

• Synced through signed-off, conflict-free merge commits.
• The PR-owned diff remains limited to tests/architecture/test_semantic_development_probe.py.
• Focused validation passed: 17 Python tests, Ruff, and git diff --check.
• The previous exact-head CI run completed with no failures. Fresh CI is running on this head.

The PR remains ready for maintainer review. No merge action was taken.