refactor(hir): unify lowering context and stores

[hongjr03]

Summary

• replace lowering mixin traits and macros with one explicit LoweringCtx<Store> owning data/source stores, scope state, and diagnostics
• make the core HIR and source-map stores explicit Rust structs and replace generated type-erased Get forwarding with named accessors
• remove future arena-ID prediction in favor of parent-first allocation/fill and pair HIR/source entries in the same allocation operation
• remove the now-unused proc-macro-utils crate

Verification

• cargo check --workspace
• cargo test -p hir (95 passed)
• cargo test -p ide --lib (140 passed)
• cargo test --workspace (532 passed, 3 ignored)