Installs the dispatch infrastructure without retiring
MethodNameCollision yet:
- 2 new CheckError variants (AmbiguousMethodResolution, UnknownClass)
- NoInstance gains optional candidate_classes field
- ResidualConstraint gains optional candidates: BTreeSet<String>
- Env gains workspace-flat method_to_candidate_classes index
- New resolve_method_dispatch helper implementing the 5-step rule
- Synth Var-arm rewritten to route through the helper
- Constraint-discharge refines multi-candidate residuals
(concrete-type path → registry filter; rigid-var path →
declared-constraint filter)
- Mono's residual-to-target mapping mirrors the discharge refinement
End-to-end multi-class fixtures still gated by MethodNameCollision
(iter mq.3); new path exercised in this iter only by 6 unit-test
cases on resolve_method_dispatch + 3 each on refine and mono
resolver.
Boss decisions on recon open questions: residual extension via
Option<BTreeSet<String>> field; index lives workspace-flat on Env;
NoInstance.candidate_classes wired through to_diagnostic this iter;
qualify_class_ref_in_check consolidation deferred per mq.1
known-debt.
