Type-store-issued handles for nominal types and indexes #1942

Summary

Nominal types (constructors) and indexes are named in HIR and CheckedType by resolved names (ResolvedConstructorName, ResolvedIndexName via IndexTypeRef::Declared), and later stages look them up in the project type store. Those lookups can miss in principle and return internal errors.

Remaining sites

• Nominal: generic_substitution.rs (constructor and field-semantics lookups), tir/typed/collect.rs, concrete_obligations.rs and dim_check/mod.rs (owner DagId → DAG).
• SortedGenericArgs: CheckedType::Struct carries unvalidated generic arguments; it is used as an Eq/Hash type identity with about 15 construction sites.
• Index axes (compile-side B2): expression_axes.rs, infer/hir/indexing.rs, recurrence.rs, prepare/model_schema.rs.

Why it was left out

HIR is finished per module before the project type store exists, so a store-issued handle needs a new completeness stage between HIR and the store. Nominals are shared across programs, so a DAG position cannot live on ResolvedNominal. Changing CheckedType::Struct changes a core type identity.

Proposed direction

Design the completeness stage first (a design doc), then introduce store-issued handles for constructors and indexes, then migrate the sites above. Expect this to be split into several PRs.