fix(learning-harness): the round-4 review findings

Three defects, two of them the same shape as ones already rejected once:

- The meta-agent proposer still could not propose. `MetaAgent(inner_model=)`
  takes a ModelConfig; it was handed a model name, a `# type: ignore` hid
  that from mypy, and a bare `except Exception` turned the resulting
  AttributeError into `meta_agent_failed` -- which reads exactly like the
  model declining. The composition is now a seam a test can pin, and a
  construction error is re-raised rather than reported as a quiet round.
- `ousast pairs --hunter` crashed in its documented form: the K floor landed
  with `--k-runs` still defaulting to 1. The covering test always passed
  `--k-runs 4`, which is why nothing noticed.
- 28 rows (12% of the corpus) resolved a span on the vulnerable side and
  none on the fixed side, so every fixed-side finding read as outside the
  labeled function and the pair scored as silence whatever the detector
  said. `fix_function` now reaches the catalog and the scorer, and the
  generator derives it from the fixed excerpt for the 25 trap rows that
  never declared one. Measured after: 0 rows.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

.kiro/specs/learning-harness/tasks.md

@@ -238,3 +238,8 @@
   - **The fixture-difficulty check** (`learning/difficulty.py`), on three axes the corpus really varies on: binding hops from the identity source to the constrained use, registration distance (0 decorator on the handler, 1 statement elsewhere in the file, 2 another document), and guard distance (0 decorator, 1 call in the body, 2 nothing visible). The comparison is per axis over the whole set, because a suite covers a bug shape with several handlers between them. **Measured first, and it failed: the shared obligations fixture reached (1, 0, 2) where the vibe-py absence rows reach (5, 2, 2).** Every shared handler carried its own route decorator, so nothing in the suite exercised a handler registered from another document and no chain was longer than one binding — the checker was passing on a simpler world than the one it meets. `from_spec` was added to the fixture: the connexion shape those rows really have, a token validated by a call rather than a decorator and five bindings between the token and the constraint. The verdict is committed as `benchmarks/measurements/2026-09-06-fixture-difficulty.json` with both reaches and the history of the first measurement. One existing expectation moved with it (`from_spec` reads the user row and then the book: two real operations).
 - Task 4.3 (2026-09-06, RED first: 10 failing tests in `tests/test_learning_cutoff.py`; then green, 11). The catalog generator fills `fix_date` from the recipe at the precision the recipe has: a full ISO day when one was recorded, otherwise the year. Every harvested recipe carries a year and almost none carries a day, and writing `-01-01` would turn a year into a claim about a month. A row therefore counts as post-cutoff only when its **earliest possible** date is after the cutoff, so a coarse date can under-claim the clean slice but never over-claim it — and over-claiming is the direction that would turn contamination into a result. `PairFamilyScore` carries the date, `aggregate(cutoff=...)` reports the same numbers over the post-cutoff rows beside the whole number and never instead of it, undated rows are counted on their own, and `publish` prints the slice with the cutoff it used. Without a configured cutoff every scoreboard says so rather than inventing one; `[learning].cutoff_date` has no default because guessing a model's training cutoff and publishing a contamination number from the guess would be worse than reporting none. Corpus dates as harvested: vibe-py all 2026, agent-vfc 67 of 69 in 2026, vfc spread 2014-2026, vfc-js 2014-2021.
 - Task 5.1 (2026-09-06, RED first: 3 of 9 tests in `tests/test_degradation_matrix.py`; then green, 8 after one test was deleted rather than kept). The three gate outputs are captured at `71efc9c` — the commit before the first line of this feature — as `benchmarks/measurements/2026-09-06-gate-baseline.txt`, and a test reproduces them byte for byte on every run, in both matrices. Until now byte-identity was something a reviewer checked by hand against a worktree; it is now mechanical and runs in CI, which is what makes it a gate rather than a habit. Six absences are each exercised once with the reason they record: HarnessX (`harnessx_unavailable`), the chat endpoint (`learning_endpoint_unavailable`, and the run still writes its artifact), the family configurations (`learning_configs_unavailable`), the network for pointer pairs (`pointer_pair_skipped`), the semantic extra (`language_unsupported` rather than a guess), and the sandbox (`sandbox_unavailable`, so a claim nothing could run is not confused with a claim that failed). One test written for this task was **deleted before the commit**: it asserted that this file contained a `without_<capability>` name per capability, which measures the file's own naming and nothing about the code.
+- Review round 4 (2026-09-06) APPROVED 4.1, 4.2, 4.3; REJECTED the round-3 remediation on three counts. The reviewer re-derived the RED phase for group 4 (24 of 26 for 4.1, 3 for 4.2, 11 of 11 for 4.3 — **every count in the notes above was understated**, corrected here), killed 18 of 19 mutants, verified the 4.1 observable independently (2 of 10 access-control rows named at `7a33756`, 9 of 10 at `3e71e9e`, the tenth stating its reason), confirmed `_python_route_access` changed no access level anywhere in the corpus and `.mjs` changed exactly one snapshot, and reproduced the fixture-difficulty artifact number for number including its failing arm. Remediated RED-first:
+  - **The meta-agent proposer still could not propose.** `MetaAgent(inner_model=...)` takes a `ModelConfig`; it was handed a model name, a `# type: ignore` hid that from mypy, and a bare `except Exception` turned the resulting `AttributeError` into the degradation reason `meta_agent_failed` — which reads exactly like the model declining. Every other HarnessX composition site in the repository builds the config properly. `_meta_agent_class` and `_model_config` are now seams a test can pin without the extra, and a `TypeError`/`AttributeError`/`ImportError` from the call is re-raised: "we called the agent wrong" is not "the agent had nothing to propose".
+  - **`ousast pairs --hunter` crashed in its documented form.** The K floor landed with `--k-runs` still defaulting to 1, so the flag's absence raised `ValueError` from inside `evaluate_catalog`. The default is `None` (three), and a breach exits naming the flag. The covering test always passed `--k-runs 4`, which is why nothing noticed.
+  - **28 rows resolved a span on the vulnerable side and none on the fixed side** (12% of the corpus), so every fixed-side finding read as outside the labeled function and the pair scored as silence whatever the detector said — the same flattering direction as the range defect, one-seventh the size. `fix_function` reached only the harvester; it now reaches the catalog and `PairCase`, `_spans` uses it for the fixed side, and the catalog generator derives it from the fixed excerpt itself for the 25 vibe-py trap rows that never declared one. A row that still resolves no fixed-side span reports `unresolved_label:fixed` instead of scoring silent. Measured after: **0 rows**.
+  - Suggestions applied: one spelling per corpus fact everywhere (`PairCase.unscorable` verbatim, no invented prefix); leave-one-out records non-teachers withheld for tier or an unscorable reason, not only holdout refusals; the hygiene test asserts the declared twins still hash equal, so a declaration that stopped being true would not be invisible; `is_post_cutoff` refuses a cutoff coarser than a day, which was the one over-claim path that existed, and month precision is pinned; `UNSCORABLE_REASONS` covers the whole vocabulary rather than `identical_twin` alone; the zero-run score carries its date; `_pairs_named_by` filters by substring before the regex.