test(ratchet): nothing fails when a file-size ceiling is raised — only new entries are guarded #16779

Problem

repo_tests/python_file_size_ratchet_test.py:17 states the rule: "the list may not gain an entry, and no ceiling may be raised (the obvious one)". Only the first half is enforced.

MAX_KNOWN_LARGE_ENTRIES = 497 (:95) is a cardinality guard — it denies a new entry appearing in KNOWN_LARGE and RATCHET_BASELINE together, the two-sided addition described at :78-85. Nothing checks an existing ceiling against its previous value.

test_recorded_ceilings_match_the_files_today (:142) asserts audit_ceilings() returns no problems — the ceilings must match the files as committed. So the passing condition for a file that grew is to edit its baseline to the new size. Growth plus a matching baseline edit is green; the guard cannot tell it from a legitimate re-measure.

Observed on PR #16778, where the rebase raised two ceilings:

-    "autobot-backend/api/knowledge_maintenance.py": 1979,
+    "autobot-backend/api/knowledge_maintenance.py": 1980,
-    "autobot-backend/api/knowledge_search_aggregator.py": 951,
+    "autobot-backend/api/knowledge_search_aggregator.py": 979,

CI does not object. The rule held only because a reviewer read the baseline diff — which is exactly the failure mode :78-85 was written about, one level up: at two raised entries it is visible in a diff, at fifty it is not.

Acceptance criteria

• A test compares each entry's ceiling against the same entry's value in the merge base and fails when any ceiling is higher.
• The comparison reads the base from git (the file's committed state), not from a second checked-in copy that would itself need a guard.
• A deliberate raise is possible but must be explicit and reviewable — an allowlist entry naming the file and the issue that sanctioned it, not a silent number change.
• Lowering a ceiling stays free and needs no ceremony.
• The test fails on a synthetic raise, proving it looks — an always-passing version of this guard is the defect it is meant to prevent.
• The duplicated baseline in scripts/python_file_size_known_large.py is covered by the same check, or the duplication is removed.

Related

The same class of gap as #16354 and #15459: a guard whose documented rule is stronger than its assertion. See docs/developer/RATCHET_BASELINES.md and docs/developer/MEASUREMENT_DISCIPLINE.md.