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

src/openultrasast/config.py

@@ -143,6 +143,18 @@ class VariantsConfig:
     max_mechanisms: int = 500
 
 
+@dataclass(frozen=True)
+class ModelLayerConfig:
+    # contributor-scan: the model layer in MAP. Named for the LAYER, not the model: `ModelConfig` is already
+    # the `[models]` block (hunter, judge, endpoint) and two meanings of one name is how you get a scan that
+    # silently reads the wrong settings. Additive -- without a CPG engine the scan is unchanged.
+    enabled: bool = True
+    # A TOTAL for the run, not a per-region cap. Conservative by default: a first scan of an unfamiliar
+    # repository should not be able to run away with someone's budget before they have decided they want it.
+    max_model_calls: int = 100
+    max_regions: int = 300
+
+
 @dataclass(frozen=True)
 class ObligationsConfig:
     # authorization-obligations: the obligation checker in MAP (standard/deep only).
@@ -168,6 +180,7 @@ class ResolvedConfig:
     regress: RegressConfig = RegressConfig()
     variants: VariantsConfig = VariantsConfig()
     obligations: ObligationsConfig = ObligationsConfig()
+    model: ModelLayerConfig = ModelLayerConfig()
     runs_dir: str = ".openultrasast/runs"
 
 
@@ -194,6 +207,7 @@ def load_config(config_path: Path | None = None) -> ResolvedConfig:
         regress=_load_regress(data.get("regress", {})),
         variants=_load_variants(data.get("variants", {})),
         obligations=_load_obligations(data.get("obligations", {})),
+        model=_load_model(data.get("model", {})),
         runs_dir=os.environ.get("OPENULTRASAST_RUNS_DIR", ".openultrasast/runs"),
     )
 
@@ -369,6 +383,15 @@ def _load_variants(value: object) -> VariantsConfig:
     )
 
 
+def _load_model(value: object) -> ModelLayerConfig:
+    data = _section(value)
+    return ModelLayerConfig(
+        enabled=bool(data.get("enabled", ModelLayerConfig.enabled)),
+        max_model_calls=_int_value(data.get("max_model_calls"), ModelLayerConfig.max_model_calls),
+        max_regions=_int_value(data.get("max_regions"), ModelLayerConfig.max_regions),
+    )
+
+
 def _load_obligations(value: object) -> ObligationsConfig:
     data = _section(value)
     return ObligationsConfig(