feat(learn): A/B experiment runner -- registered manifests, paired arms, bootstrap and McNemar (decision-engine task 7)

learn/experiments.py: YAML manifests under plane/experiments/<id>.yaml,
digest-checked at register, run and analyse time; `experiment`,
`arm_outcome` and `experiment_result` rows; paired arms in a seeded
repository order with a per-unit coin; arm A replay-only at $0; canary
replicates under a fresh cache salt; repository-cluster paired bootstrap,
exact McNemar, two O'Brien-Fleming looks, adopt/reject/equivalent/
inconclusive. `ousast learn experiment register|units|run|analyse`.
Program: `ProgramSpec.sampling = retrieval_ensemble` (k samples at
temperature 0 over k disjoint retrieved example sets; default unchanged);
`Caller.salt`; `retrieve(exclude=)`. verify: OUSAST_VERIFY_CALLERS=off.
Manifests exp-001-known-callers (plane arms, units pending) and
exp-002-retrieval-ensemble (slice-2 incumbents, ceiling $4) -- not run.
18 scripted tests; module audit regenerated (151 modules).

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

tests/test_plane_increment.py

@@ -36,7 +36,7 @@ def _mebibytes(quantity: str) -> float:
 
 
 def test_every_manifest_under_plane_loads() -> None:
-    paths = sorted(PLANE.rglob("*.yaml"))
+    paths = sorted(p for p in PLANE.rglob("*.yaml") if "experiments" not in p.parts)  # experiment manifests are not ax manifests
     assert len(paths) >= 36, paths
     manifests = load_manifests(paths)
     assert set(manifests.models) == {"deepseek-flash", "openrouter-embedding"}  # the second: embeddings only (decision engine)