test(learning-harness): apply the group 1 review suggestions

A mechanism claimed by two families now fails as loudly as a weakness claimed
twice, with a named offender. Two behaviours that were correct but unasserted
are pinned: the obligations tool refuses a path outside the repository, and a
provider that refuses to build degrades to no endpoint instead of raising into
a scan. The TypeScript parse assertion is now exact rather than a quarter-of-
the-slice allowance, which surfaces one real corpus defect: a TSX excerpt
harvested as a truncated destructuring parameter list is not valid TypeScript
on its own, and belongs to the re-harvest rather than to the grammar.

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

src/openultrasast/learning/families.py

@@ -99,6 +99,7 @@ def load_families(path: Path | None = None, *, require_all: bool = True) -> Fami
         raise FamiliesError(f"{source}: version is required")
     families: list[Family] = []
     claimed: dict[str, str] = {}
+    claimed_mechanisms: dict[str, str] = {}
     for entry in payload.get("family", []):
         if not isinstance(entry, dict):
             raise FamiliesError(f"{source}: every [[family]] entry must be a table")
@@ -109,6 +110,12 @@ def load_families(path: Path | None = None, *, require_all: bool = True) -> Fami
             if cwe in claimed:
                 raise FamiliesError(f"{source}: {cwe} is claimed by both {claimed[cwe]!r} and {family.id!r}")
             claimed[cwe] = family.id
+        for mechanism in sorted(family.mechanisms):
+            if mechanism in claimed_mechanisms:
+                raise FamiliesError(
+                    f"{source}: mechanism {mechanism} is claimed by both {claimed_mechanisms[mechanism]!r} and {family.id!r}"
+                )
+            claimed_mechanisms[mechanism] = family.id
         families.append(family)
     _check_parents(source, families)
     if require_all and tuple(family.id for family in families) != FAMILY_IDS: