feat(learning-harness): declared twins and a fixture-difficulty check

The five byte-identical vibe-py twins now say so in the catalog instead of
only hashing to it, and a declared limit that names one of the unscorable
vocabulary's own reasons keeps that spelling, so `identical_twin` stays one
fact. Leave-one-out over vibe-py consequently runs on 30 rows, not 35.

The difficulty check measures three axes the corpus varies on: binding hops
from the identity source to the constrained use, registration distance, and
guard distance. Measured first, and it failed: the shared obligations
fixture reached (1, 0, 2) where the vibe-py absence rows reach (5, 2, 2).
Every fixture handler carried its own route decorator, so nothing in the
suite exercised a handler registered from another document and no binding
chain was longer than one hop -- the checker was passing on a simpler world
than the one it meets. `from_spec` closes that with the connexion shape
those rows really have. The verdict is committed beside the numbers.

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

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

@@ -143,7 +143,7 @@
   - _Depends: 1.3_
   - _Requirements: 10.2_
 
-- [ ] 4.2 Identical twins and the fixture-difficulty check
+- [x] 4.2 Identical twins and the fixture-difficulty check
   - The five identical vibe-py twins carry the identical-twin known-limit reason in the catalog; a test derives a difficulty vector for a unit fixture (binding hops to the identity source, registration present, cross-file registration) and for the corpus rows its docstring names, and fails when the fixture is strictly easier.
   - Observable: the twins are excluded from every denominator with their reason listed; the shared obligations fixture is compared against the re-harvested vampi rows and the check's verdict is recorded.
   - _Depends: 4.1_
@@ -233,3 +233,6 @@
   - **3.1, refusals were `continue` statements.** `Refusal.degradation()` existed and nothing called it. The mechanism lever now records every candidate it declined with the holdout pairs that taught it, leave-one-out records the pairs it kept out of the teaching set, and `improve` carries both into the round outcome. `semantic.seed` reported `split:train` for a train-split identical twin, which is simply untrue; an unscorable row now reports its own reason. `evaluate_profiles` and `evaluate_mechanism_profiles` are deliberately split-blind and now say so in their docstrings: they measure a ruleset and never learn from a pair, so a filter there would only shrink the evidence.
   - Test-quality repairs the review asked for, each of which changed a number: two split tests were measuring an `x = 1` fixture that teaches nothing whatever the rule says, so both arms now use a pair that really carries a shape and the lever test asserts the same shape *is* admitted when a train pair taught it; the two CLI scoreboard tests asserted against output that was all zeros, and now use a side-aware detector stub (the shipped `scripted` stub answers twice, then goes quiet, and puts its finding outside the labeled function — three separate ways to make a family number zero); `ScriptedChatClient` gained an opt-in `cycle` so a stub cannot silently run out of answers mid-run; the sandbox-candidate assertion in `tests/test_learning_scan.py` compared verdict paths against a prefix no implementation would produce and now compares finding ids; the manifest's classifier block reported `measure_classifier([])`, all zeros, and now reports what this scan actually routed.
   - Carry-forward, unchanged and still capping every holdout number: the nineteen straddling repositories, and — found during task 4.1 — **nine of the ten duplicate groups sit on both sides of the split**, which the repository-level split cannot see because the duplicates are different repositories.
+- Task 4.2 (2026-09-06, RED first: 2 failing tests in `tests/test_fixture_difficulty.py`, and the check's own arm — `test_the_check_catches_a_fixture_that_is_easier_on_one_axis` — fails if the fixture hardening is reverted). Two parts:
+  - **The five twins declare their limit.** A content hash is evidence; the catalog row is the maintainer standing behind it, and without one a re-harvest that changes a byte turns a known-unscorable row back into a scored one with no review. A declared limit that names one of the vocabulary's own reasons keeps that spelling, so `identical_twin` declared and `identical_twin` computed stay one fact instead of splitting across two buckets in every aggregation. `semantic.seed._skip_reason` was reordered for the same reason: an unscorable row reports `unscorable:<reason>` before either the known-limit or the split branch can claim it. Consequence, recorded because it moves a published number: leave-one-out over vibe-py now runs on **30 rows, not 35** — a pair whose two sides are the same code cannot be held out against itself.
+  - **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).