[JAORMX]

Summary

• preserve underscores from inline-code heading spans in canonical anchor slugs
• allocate duplicate suffixes in the corrected slug namespace
• add local, cross-document, duplicate, mixed-inline, Unicode-context, and missing-anchor coverage
• document the anchor dialect compatibility rule

Fixes #40.

Verification

• go test -race -count=1 ./internal/infrastructure/mdparser ./internal/domain/reference ./internal/application ./cmd/matlatl
• go vet ./internal/infrastructure/mdparser ./internal/domain/reference ./internal/application ./cmd/matlatl
• task dogfood
• exact issue reproduction with matlatl check --strict

go test ./... remains blocked by pre-existing Nimbus fixture-freeze failures in eval/internal/nimbus (freeze.json does not match current Nimbus tree).