refactor(hir): remove dead scaffolding and dedup scope queries

[hongjr03]

Summary

Cleanup follow-up to the merged semantic-foundations stack (#289/#290/#288), addressing the maintainability issues raised in review. Three independent commits, each behavior-preserving.

1. Drop dead DefKind variants

DefId::kind() never produced 11 variants (Class, Covergroup, Checker, Modport, ClockingBlock, Sequence, Property, ClassField, Method, Struct, Enum); they only sat in match arms returning Unknown/empty, which reads as "intentionally unsupported" when it is actually "not yet implemented". DefKind is already #[non_exhaustive], so variants can be reintroduced together with their lowering later and the compiler will force every in-crate match to update then.

2. Drop dead Ty variants and SpecArg

Ty::ClassHandle and Ty::VirtualInterface are never constructed anywhere; they only appear in match arms returning Unknown/empty, alongside the SpecArg enum that exists solely to feed ClassHandle. Class/interface specialization is deferred, so these are forward scaffolding.

3. Deduplicate scope query member insertion

module/generate_block/block/subroutine scope queries each repeated the same decl/typedef/stmt-label insertion loops. Extracted into insert_decls_and_typedefs and insert_stmts so a new declaration category is added in one place, not four. Insertion order preserved (the two helpers bracket the container-specific members).

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace (483 passed, unchanged from baseline)

Notes

Each commit passes the full gate independently. Deliberately left untouched: the unreachable BuiltinDataTy::Event|Chandle|Void arms inside the Ty::Builtin matches of type_class/packed_bit_width — those types are promoted out of Ty::Builtin during normalization, so the arms are unreachable, but removing them would need a _ wildcard (losing exhaustive coverage of BuiltinDataTy) or a larger representation unification; both are poorer tradeoffs than leaving the defensive arms. Flagging it here so it is a conscious decision, not an oversight.