fix(learning-harness): the round-4 review findings

Three defects, two of them the same shape as ones already rejected once:

- The meta-agent proposer still could not propose. `MetaAgent(inner_model=)`
  takes a ModelConfig; it was handed a model name, a `# type: ignore` hid
  that from mypy, and a bare `except Exception` turned the resulting
  AttributeError into `meta_agent_failed` -- which reads exactly like the
  model declining. The composition is now a seam a test can pin, and a
  construction error is re-raised rather than reported as a quiet round.
- `ousast pairs --hunter` crashed in its documented form: the K floor landed
  with `--k-runs` still defaulting to 1. The covering test always passed
  `--k-runs 4`, which is why nothing noticed.
- 28 rows (12% of the corpus) resolved a span on the vulnerable side and
  none on the fixed side, so every fixed-side finding read as outside the
  labeled function and the pair scored as silence whatever the detector
  said. `fix_function` now reaches the catalog and the scorer, and the
  generator derives it from the fixed excerpt for the 25 trap rows that
  never declared one. Measured after: 0 rows.

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

src/openultrasast/learning/rounds.py

@@ -457,11 +457,13 @@ def _pairs_named_by(proposal: Proposal, cases: Sequence[PairCase]) -> tuple[str,
     A memory entry that quotes a held-out pair is the loop learning the answer instead of the shape, and it looks
     exactly like a legitimate hard negative until the holdout number is read as if it meant something."""
     haystack = "\n".join([*proposal.predicted_affected, *proposal.predicted_at_risk, *proposal.change.values()])
-    named = []
-    for case in cases:
-        name = str(getattr(case, "name", ""))
-        if name and re.search(rf"(?<![\w-]){re.escape(name)}(?![\w-])", haystack):
-            named.append(name)
+    # Substring first, regex only for the handful that survive it: the corpus is a few hundred rows and a round
+    # runs this once, but a per-round sweep of one regex per row is the kind of thing that stops being free.
+    named = [
+        name
+        for case in cases
+        if (name := str(getattr(case, "name", ""))) and name in haystack and re.search(rf"(?<![\w-]){re.escape(name)}(?![\w-])", haystack)
+    ]
     return tuple(sorted(set(named)))