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

tests/test_loo.py

@@ -103,7 +103,9 @@ def test_cli_pairs_loo_writes_the_artifact_offline(tmp_path: Path, monkeypatch:
     with redirect_stdout(buf):
         assert main(["pairs", "--slice", "vibe-py", "--loo", "--json", "--loo-out", str(tmp_path / "loo.json")]) == 0
     payload = json.loads(buf.getvalue())
-    assert payload["per_slice"]["vibe-py"]["pairs"] == 35 and "found_by" in payload["outcomes"][0]
+    # 35 vendored rows less the five byte-identical twins, which task 4.2 declared unscorable in the catalog: a
+    # pair whose two sides are the same code cannot be held out against itself.
+    assert payload["per_slice"]["vibe-py"]["pairs"] == 30 and "found_by" in payload["outcomes"][0]
     assert json.loads((tmp_path / "loo.json").read_text()) == payload