feat(model-grounded-detection): tasks 2.1-2.2, the Joern seam and the taint verdict

The whole engine integration is one subprocess boundary: joern-parse builds a
cpg.bin, joern --script runs a shipped CPGQL query that prints its JSON between
fence markers, because Joern's stdout carries a banner and a REPL prompt around
whatever a script emits. Nothing is imported in-process, so the core install
keeps dependencies = [] -- a test walks the seam's AST to assert it imports
only the standard library and names no JVM binding.

Fail closed everywhere. A build or query that errors, times out, or prints
something unparseable yields no verdict and the caller reports suspicion. The
taint verdict distinguishes this from absence deliberately: a query returning
None means "the engine could not decide", which is not "there is no flow", and
conflating them is how a missing engine silently becomes a clean bill of health.

The verdict is where the ladder earns its keep. An unsanitized source->sink
path is model_entailed -- the graph establishes it and the LLM's claim is no
longer load-bearing. A path a sanitizer may break is model_corroborated: the
flow is real, but whether the sanitizer suffices is the semantic residual the
LLM answers, checked against this verdict rather than averaged over runs. The
chosen flow is picked by a total order (shortest, then lexicographic), so two
runs over one CPG cannot disagree -- Req 6.4 asserted directly.

The query itself is the part our flat IR could not do: reachableByFlows walks
the PDG, so `q = "..." + request.args["x"]; execute(q)` is found, which is
exactly the one-hop limit that held measured entailment at 2.2%.

Capability detection reuses semantic/engines.joern_available rather than
growing a second notion of "installed". The audit manifest is regenerated; the
new modules are named by Req 4/5/6 rather than counted as orphans.

Task 2.3, the go/no-go measurement, needs Joern installed and has not run.

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

src/openultrasast/model/taint.py

@@ -0,0 +1,73 @@
+"""Taint reachability over the CPG: the arbiter for the flow families (Req 6.1).
+
+The verdict is the static analogue of a reproduced crash. An unsanitized source→sink path is `ENTAILED` — the
+graph establishes it and the LLM's claim is no longer load-bearing. A path a sanitizer may break is
+`CORROBORATED`: the flow is real, but whether the sanitizer is *sufficient* is a semantic judgement the model
+cannot make, and that residual is the LLM's — checked against this verdict rather than averaged over runs.
+
+No path at all yields `None`, not a negative verdict: the caller reports `suspicion`. And a query that failed
+yields `None` too, because "the engine could not decide" is not "there is no flow" — conflating them is how a
+missing engine would silently become a clean bill of health.
+"""
+
+from __future__ import annotations
+
+from collections.abc import Mapping, Sequence
+
+from ..cpg.backend import CpgResult
+from .ladder import Rung, Verdict
+from .specs import TaintSpec
+
+
+def verdict(cpg: CpgResult, spec: TaintSpec, *, function: str = "") -> Verdict | None:
+    """The strongest verdict the taint query supports for ``spec`` in ``function``, or ``None`` to stay at suspicion."""
+    rows = cpg.run(
+        "taint",
+        {"sources": spec.sources, "sinks": spec.sinks, "sanitizers": spec.sanitizers, "function": function},
+    )
+    flows = _flows(rows, function=function)
+    if not flows:
+        return None
+    unsanitized = [flow for flow in flows if not flow["sanitized"]]
+    # Deterministic by construction: sort, then take the first. Nothing here depends on engine ordering.
+    chosen = min(unsanitized or flows, key=_rank)
+    rung = Rung.ENTAILED if not chosen["sanitized"] else Rung.CORROBORATED
+    return Verdict(rung=rung, family=spec.family, witness=_witness(chosen))
+
+
+def _flows(rows: object, *, function: str) -> list[dict[str, object]]:
+    """The query's rows, filtered to the labeled function. A non-list (a failure) yields nothing."""
+    if not isinstance(rows, Sequence) or isinstance(rows, (str, bytes)):
+        return []
+    kept: list[dict[str, object]] = []
+    for row in rows:
+        if not isinstance(row, Mapping):
+            continue
+        if function and str(row.get("sinkMethod", "")) != function:
+            continue
+        kept.append({
+            "sink": str(row.get("sink", "")),
+            "sinkLine": str(row.get("sinkLine", "")),
+            "sinkMethod": str(row.get("sinkMethod", "")),
+            "source": str(row.get("source", "")),
+            "sanitized": bool(row.get("sanitized", False)),
+            "length": _as_int(row.get("length")),
+        })
+    return kept
+
+
+def _as_int(value: object) -> int:
+    return int(value) if isinstance(value, int | float) else 0
+
+
+def _rank(flow: Mapping[str, object]) -> tuple[int, str, str]:
+    """Shortest flow first, then lexicographic — a total order, so two runs cannot disagree."""
+    return (_as_int(flow["length"]), str(flow["source"]), str(flow["sink"]))
+
+
+def _witness(flow: Mapping[str, object]) -> str:
+    line = flow.get("sinkLine") or "?"
+    return f"{flow['source']} -> {flow['sink']} (line {line}, {flow['length']} steps)"
+
+
+__all__ = ["verdict"]