feat(mir): validate nominal and link metadata

[sanohiro]

Summary

Add the am-n HIR-to-MIR preflight for nominal and linker metadata. The validator now checks
combined internal-name uniqueness, source-name kind and recursively id-free shape equality,
member names, tuple interning uniqueness, alignment, enum field bases, and link-library names.
All four lowering entrypoints fail closed to canonical empty MIR for invalid handcrafted HIR while
valid MIR remains unchanged. Add the shared-DAG correspondence regression, deep source-shape
coverage, and the library-boundary benchmark row.

Validation

• scripts/pre-pr.sh --base origin/main ... passed, including scripts/test-pr.sh.
• cargo test -p align_mir --lib: 44 passed.
• cargo build --workspace --locked: passed.
• cargo clippy --workspace --all-targets --locked -- -D warnings: passed.
• align_driver per-unit codegen passed 9/9 before the final cache-only follow-up; the latest
  rerun compiled successfully but was interrupted after the test binary paused in macOS
  _dyld_start.
• The benchmark row is implemented but the release harness cannot run on this host because
  llvm-config-22 is unavailable.

Review

• Fresh independent adversarial preflight on the final HEAD/base pair: CLEAN.
• Align compiler self-review gates completed.