feat: refine product return provenance

[sanohiro]

Summary

• preserve exact struct and tuple member provenance through construction, projection, mutation, destructuring, and control-flow joins
• validate source-order value snapshots at owner-generation and eager action boundaries while keeping arrays, pipelines, and tags conservative
• fail closed on malformed checked HIR and preserve whole-program/per-unit summary parity
• record the L2b-a2-s implementation closure matrix and completion evidence

Verification

• scripts/test-pr.sh
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test -p align_driver --test return_provenance --test per_unit
• cargo test -p align_sema projected_return_provenance_fails_closed
• summary inference: 3.147 ms/check, 22,848 bytes/check
• import validation: 1.844 ms/import