iter mq.tidy: close 4 actionable drift items from audit-mq

T1 [high-1]: refine_multi_candidate_residual rigid-var filter now
requires class + type-unification via constraint_type_matches, so
forall a b. prelude.Show a, userlib.Show b => ... shapes discriminate
by which typevar the residual is on. Discharge-time only; synth-time
twin doesn't have a residual type to unify against (fresh metavar
constructed after dispatch).

T2 [high-2]: qualifier_is_class_shape extracted as pub(crate) helper
adjacent to parse_method_qualifier; broadened to accept PascalCase
single-segment qualifiers like Show.show alongside module-qualified
ones. Same-module bare-class call sites now reachable; symmetric to
mq.1 canonical-form rule.

T3 [medium-1]: any_candidate_class_has_instance pub(crate) helper
gates the class-method-shadowed-by-fn warning closure on registry
instance presence (any candidate class, any type). Conservative
tightening of the spec rule per Boss Q2 decision.

T4 [medium-2]: four DESIGN.md Data Model schema fragments
(SuperclassRef, InstanceDef.class, Type::Con.name, Constraint.class)
annotated with canonical-form cross-references.

Two trip-wires fixed inline: parse_method_qualifier docstring
coherence, mq3 in-crate test fixture-repair (registry instance
injection — analogous to mq.3 typeclass_22b3 pattern).

5/5 tasks. 548 tests green (was 545 + 3 new mq_tidy_* unit tests).
bench/compile_check.py + cross_lang.py exit 0; check.py exit 0
both runs with noise-class metric migration on
latency.implicit_at_rc.* max-tail (6th-consecutive observation
since audit-cma). Baseline pristine.

docs/DESIGN.md

@@ -2136,7 +2136,7 @@ are real surface forms.
 { "kind": "class",
   "name": "<id>",          // class name (e.g. "Show")
   "param": "<id>",          // single class parameter, kind *
-  "superclass": null,       // or { "class": "<id>", "type": "<param>" }
+  "superclass": null,       // or { "class": "<id>", "type": "<param>" } — "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module; see §"Class names" / mq.1)
   "methods": [
     { "name": "<id>",
       "type": Type,         // FnSig over the class param
@@ -2149,7 +2149,7 @@ are real surface forms.
 
 // instance (typeclass instance; see Decision 11)
 { "kind": "instance",
-  "class": "<id>",          // class being instantiated
+  "class": "<id>",          // class being instantiated; canonical form (bare for same-module, "<module>.<Class>" for cross-module; see §"Class names" / mq.1)
   "type": Type,             // concrete type expression (never the class param)
   "methods": [
     { "name": "<id>", "body": Term }
@@ -2250,7 +2250,7 @@ Patterns are linear: each pattern variable may appear at most once.
 ```jsonc
 // Type-constructor application. `args` omitted when empty
 // (hash-stable when omitted, for non-parameterised cases like Int, Bool, ...).
-{ "k": "con", "name": "<id>", "args": [Type...] }
+{ "k": "con", "name": "<id>", "args": [Type...] }   // "name": canonical form (bare for same-module / primitives, "<module>.<TypeName>" for cross-module; see §"Type::Con name scoping" / ct.1)
 
 // Function type. Decision 10 added paramModes/retMode as
 // metadata on Type::Fn — they are NOT separate Type variants, so every
@@ -2270,7 +2270,7 @@ Patterns are linear: each pattern variable may appear at most once.
 // constraints (Decision 11); omitted when empty (hash-stable when omitted).
 { "k": "forall",
   "vars": ["<id>"...],
-  "constraints": [{ "class": "<id>", "type": "<id>" }, ...],
+  "constraints": [{ "class": "<id>", "type": "<id>" }, ...],   // "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module; see §"Class names" / mq.1)
   "body": Type }
 ```