fix: give error-set decls per-decl nominal identity (issue 0134)

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).

issues/0134-error-set-no-per-decl-nominal-identity-same-name-collapse.md

@@ -1,5 +1,16 @@
 # 0134 — a same-name `error` set collapses into a namespaced import's set (error sets lack per-decl nominal identity)
 
+> **RESOLVED.** Error-set declarations now get the same per-decl nominal
+> identity (E6a) as struct/enum/union. `registerErrorSetDecl` builds the
+> `.error_set` `TypeInfo` (via a new `buildErrorSetInfo` helper factored out of
+> `resolveInlineErrorSet`) and interns it through `internNamedTypeDecl` with a
+> `shadowNominalId`; a `reserveShadowErrorSetSlot` reserves a distinct slot in
+> `scanDecls`, and `namedRefTid`'s `.error_set_decl` arm consults the per-decl
+> `type_decl_tids` before falling back to `findByName` — so a local set no
+> longer collapses onto a same-name imported one. The inline/anonymous
+> `findByName` short-circuit is preserved. Regression test:
+> `examples/1059-errors-same-name-error-set-own-wins.sx`.
+
 ## Symptom
 
 One-line: a top-level `error { ... }` whose NAME matches an error set