feat(model-grounded-detection): group 6, the execution tier as a seam not a build

The rung this project deliberately does not build. Clearwing already runs
sandbox lifecycles, container pools, sanitizer images, PoC replay and stability
classification, and the corrective brief concluded -- from this project's own
defect history -- that reimplementing that machinery is where we go wrong. So
model/execution.py decides eligibility, calls an adopted tool, and records what
happened. A test asserts structurally that it imports no container or
subprocess machinery and defines no build/run/compile/patch function, because
the failure mode to prevent is this file quietly growing a sandbox.

Eligibility is narrow in both directions: only a family a static model cannot
arbitrate (memory safety in C, where the bug turns on runtime layout -- Req
10.3 forbids any web/logic family depending on this tier, and each now has
taint, dominance or constant abstraction instead), and only a finding the model
left undecided. The tier escalates uncertainty; it never re-checks a verdict.

One deliberate asymmetry: a failure to reproduce is NOT a refutation. A sandbox
that does not trigger a bug has not shown the bug is absent, only that this
attempt did not trigger it, so the candidate stays where the model left it.
Reading a failed reproduction as a clean bill of health is the same error class
as reading an absent engine as "no vulnerability".

With no adopted tool present -- the normal case -- the rung is never reached and
clearwing_unavailable is recorded.

All 16 tasks of model-grounded-detection are now done. 720 passing, mypy/ruff
clean, gates byte-identical, zero orphaned modules.

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

src/openultrasast/model/audit.py

@@ -38,6 +38,7 @@ SPEC_OWNED = {
     "model.config_value": "model-grounded-detection Req 6.3: constant abstraction for the configuration families",
     "model.calibrate": "model-grounded-detection Req 9: the corpus as the model's calibration set",
     "model.report": "model-grounded-detection Req 9.3/11.1: per-slice reporting with the overfitting gap",
+    "model.execution": "model-grounded-detection Req 10: the deferred execution tier seam, adopted not built",
     "cpg": "model-grounded-detection Req 4: the CPG seam package",
     "cpg.backend": "model-grounded-detection Req 4.1/4.5: the single Joern subprocess boundary",
     "cpg.capability": "model-grounded-detection Req 4.2/4.3: the engine probe that degrades to suspicion",