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

benchmarks/measurements/2026-09-08-module-audit.json

@@ -1,7 +1,7 @@
 {
-  "modules": 150,
+  "modules": 151,
   "counts": {
-    "load_bearing": 145,
+    "load_bearing": 146,
     "standalone_capability": 5
   },
   "orphaned": [],
@@ -355,6 +355,7 @@
         "learn.evaluate",
         "learn.examples",
         "learn.excerpt",
+        "learn.experiments",
         "learn.features",
         "learn.folds",
         "learn.labels",
@@ -390,7 +391,8 @@
       "reason": "reachable from gate, map_gate, pair_gate, cli",
       "importers": [
         "learn.cli",
-        "learn.evaluate"
+        "learn.evaluate",
+        "learn.experiments"
       ]
     },
     {
@@ -408,7 +410,8 @@
       "reason": "reachable from gate, map_gate, pair_gate, cli",
       "importers": [
         "learn.cli",
-        "learn.compile"
+        "learn.compile",
+        "learn.experiments"
       ]
     },
     {
@@ -419,6 +422,7 @@
         "learn.cli",
         "learn.compile",
         "learn.evaluate",
+        "learn.experiments",
         "learn.folds",
         "learn.program",
         "learn.retrieve"
@@ -435,6 +439,14 @@
         "plane.harvest"
       ]
     },
+    {
+      "module": "learn.experiments",
+      "classification": "load_bearing",
+      "reason": "reachable from gate, map_gate, pair_gate, cli",
+      "importers": [
+        "learn.cli"
+      ]
+    },
     {
       "module": "learn.features",
       "classification": "load_bearing",
@@ -452,6 +464,7 @@
         "learn.cli",
         "learn.compile",
         "learn.evaluate",
+        "learn.experiments",
         "learn.program",
         "learn.retrieve"
       ]
@@ -487,7 +500,8 @@
       "importers": [
         "learn.cli",
         "learn.compile",
-        "learn.evaluate"
+        "learn.evaluate",
+        "learn.experiments"
       ]
     },
     {
@@ -831,6 +845,7 @@
         "learn.compile",
         "learn.embeddings",
         "learn.examples",
+        "learn.experiments",
         "learn.program",
         "plane.egress",
         "plane.engine_alerts",
@@ -855,6 +870,7 @@
         "learn.cli",
         "learn.compile",
         "learn.embeddings",
+        "learn.experiments",
         "learn.program",
         "plane.generate",
         "plane.tasks.roles",
@@ -920,7 +936,7 @@
     {
       "module": "plane.memory",
       "classification": "load_bearing",
-      "reason": "harnessx-removal Req 6.1/6.4: the memory store (file, MinIO), ingest and fact reuse; `ousast plane remember`",
+      "reason": "harnessx-removal Req 6.1/6.4: the memory store (file, S3), ingest and fact reuse; `ousast plane remember`",
       "importers": [
         "cli",
         "improve.memory",
@@ -928,6 +944,7 @@
         "learn.compile",
         "learn.embeddings",
         "learn.examples",
+        "learn.experiments",
         "learn.program",
         "plane.generate",
         "plane.tasks.loop",