feat(contributor-scan): 1.3 the model layer reaches a user

`model-grounded-detection` finished 16/16 with the model layer having zero
production callers. This closes that. A real scan of a Flask file now reports:

  by_rung: {"model_entailed": 3, "model_corroborated": 0, "suspicion": 0}
  model:injection:ping:10  <- os.system("ping -c 1 " + request.args["host"])

The stage is the fifth MAP proposer, mirroring _check_obligations, and is
additive: pattern, overlay and obligation findings all keep working, and without
a CPG engine it records cpg_unavailable and changes nothing else. Two scan
integrity tests pin the exact degradation set and both broke -- correctly. Their
`requested == baseline` assertion, that findings are byte-identical, still
passed, so the only change is that the scan now SAYS the arbiter was absent
instead of being silently without one. The guards are updated to assert the
model degradation specifically rather than being loosened.

ModelLayerConfig, not ModelConfig: that name is already the [models] block, and
the type checker caught the collision immediately. Two meanings of one name is
how a scan silently reads the wrong settings.

Every payload carries the same shape, skipped ones included. A payload that is
sometimes a full record and sometimes a single "skipped" key is how a report
ends up quietly omitting a section.

unjudged_sample names the lowest-ranked regions the budget did not reach.
Counting them is the honesty requirement; naming them is what lets a maintainer
write a large-repository exclusion list from evidence rather than guesswork.

The run also produced the finding behind new task 1.4: 275 seconds for a
ten-line file. cost_usd is 0.0, so those 21 invocations were not model calls --
they were Joern query launches, one per region per family, each a JVM start.

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

tests/test_scan_pipeline_integrity.py

@@ -65,10 +65,14 @@ def test_standard_scan_extra_absent_matches_deterministic_baseline(tmp_path: Pat
 
     assert requested == baseline  # byte-identical findings -> sync driver behaviour unchanged
     stages = {entry["stage"] for entry in requested_manifest.get("degradations", [])}
-    assert stages == {"hunter_pool", "verify"}
-    assert all(entry["reason"] == "harnessx_extra_unavailable" for entry in requested_manifest["degradations"])
+    # `model` joins the set when the model layer runs without a CPG engine: findings are byte-identical
+    # (asserted above) and the only difference is that the scan now SAYS the arbiter was unavailable
+    # (contributor-scan Req 1.3) rather than being silently absent.
+    assert stages == {"hunter_pool", "verify", "model"}
+    assert {e["reason"] for e in requested_manifest["degradations"] if e["stage"] == "model"} == {"cpg_unavailable"}
+    assert all(entry["reason"] == "harnessx_extra_unavailable" for entry in requested_manifest["degradations"] if entry["stage"] != "model")
     assert all(entry["reason"] != "hunter_model_unavailable" for entry in requested_manifest["degradations"])
-    assert [entry["reason"] for entry in baseline_manifest.get("degradations", [])] == ["hunter_model_unavailable"]
+    assert [e["reason"] for e in baseline_manifest.get("degradations", []) if e["stage"] != "model"] == ["hunter_model_unavailable"]
 
 
 def test_quick_scan_is_unaffected_by_the_extra(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None: