M2 Materialization + extractor MVP: cargo integration, crate-ir, recorded fixtures

[rahlk]

Part of #27. cargo metadata materialization + Cargo.lock hashing; extraction orchestration (RUSTC_WORKSPACE_WRAPPER under cargo check, per-crate IR in cache_dir, graceful per-crate degradation); HIR items/spans/structured attrs; impl index incl. external targets; typed call candidates (static/trait-method/fn-ptr) incl. closure bodies and macro-expanded sites; recorded-IR fixtures making core tests stable-only + drift guard. Gate: e2e suite green on the fixture.