Rigid-var refinement type-unification leg in refine_multi_candidate_residual
(high-1); same-module bare-class qualifier shape accepted (high-2);
class-method-shadowed-by-fn warning tightened to require an instance
in workspace_registry (medium-1); DESIGN.md Data Model schema fragments
cross-reference the canonical-form rule (medium-2).
Q1 revised from carrier "fix both sites" to "fix only discharge-time"
after code inspection: synth-time resolve_method_dispatch is invoked
with concrete_arg_type: None and the residual's metavar is constructed
AFTER the dispatch call, so the rigid-var leg has no residual type to
unify against — class-only filter is semantically correct there.
Q2 conservative tightening: filter via "any candidate class has any
registry instance" instead of deferring to App-arm via pending-warning
list (full spec rule).
