Iter mq.1 plan: move InstanceDef.class / Constraint.class /
SuperclassRef.class from bare-only to canonical form per ct.1's
existing rule, extend validate_canonical_type_names with three new
field walks, narrow check_class_name_fields to ClassDef.name-only,
qualify build_registry's class_def_module + class_by_name + entries
+ four lookup sites, qualify ClassMethodEntry.class_name +
class_superclasses, update affected test-assertion strings.
Pre-flight recon flagged that the spec's '14 fixtures migrated' cost
claim is over-stated: all existing fixture class refs are intra-module
under the canonical-form rule. Plan does not add migration work that
does not need doing; Task 7 Step 3 verifies zero existing-fixture
diff.
