merge: taint-query null-guard fix and trace-enabled engine runner

The query fix stops a class-level call from aborting the whole taint batch on
real PHP code; gate, map_gate and pair_gate outputs are unchanged. Trace parsing
lives with the runner (benchmarks/learn/engine_trace_parse.py).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

benchmarks/learn/engine_trace_parse.py

@@ -0,0 +1,51 @@
+"""Lossless parsing of the optional shortest-path trace, with explicit evidence limits."""
+
+from __future__ import annotations
+
+import re
+from collections.abc import Mapping
+from typing import Any
+
+_STEP = re.compile(r"^(.*):(-?\d+) \[([^\]]+)\] ([\s\S]*)$")
+
+
+def parse_trace(row: Mapping[str, Any]) -> dict[str, Any]:
+    """Preserve row-level discharge evidence; never guess which node sanitized a path.
+
+    The query emits basename-only files and truncated code. A bound can be off-path;
+    it remains in ``guard`` even when no trace step exactly matches it. Invalid steps
+    raise instead of turning an unreadable trace into an empty path.
+    """
+    raw = row.get("trace", [])
+    if not isinstance(raw, list):
+        raise ValueError("trace must be an array")
+    steps = []
+    bound = str(row.get("bound") or "")
+    for index, text in enumerate(raw):
+        match = _STEP.fullmatch(text) if isinstance(text, str) else None
+        if match is None:
+            raise ValueError(f"invalid trace step: {text!r}")
+        location, line, node_label, code = match.groups()
+        file, separator, method = location.partition(":")
+        roles = ["source"] if index == 0 else ["step"]
+        if index == len(raw) - 1:
+            roles = ["source", "sink"] if index == 0 else ["sink"]
+        if bound and code == bound:
+            roles.append("guard")
+        steps.append(
+            {
+                "file": file,
+                "method": method if separator else "?",
+                "line": int(line),
+                "node_label": node_label,
+                "code": code,
+                "roles": roles,
+                "raw": text,
+            }
+        )
+    return {
+        "steps": steps,
+        "sanitized": bool(row.get("sanitized", False)),
+        "sanitizer": {"on_path": bool(row.get("sanitized", False)), "step": None},
+        "guard": {"text": bound, "bounded": bool(row.get("bounded", False))},
+    }