A local 'error { ... }' set with the same name as an imported one collapsed
onto the import, losing its own tags, because registerErrorSetDecl deduped via
the flat findByName path while struct/enum/union use E6a per-decl identity.
Build the .error_set TypeInfo (new buildErrorSetInfo helper factored from
resolveInlineErrorSet) and intern via internNamedTypeDecl with shadowNominalId;
reserve a distinct shadow slot in scanDecls; consult per-decl type_decl_tids in
namedRefTid before findByName. The inline/anonymous findByName short-circuit is
preserved.
Regression: examples/1059-errors-same-name-error-set-own-wins.sx (moved from
issues/0134).
