Fix stored-truth category qualification and reduce passport churn on generate

[Spenquatch]

Summary

• Stop preserve-mode semantic review from backfilling seam descriptor_id values, so read-side surfaces keep stored truth instead of minting fresher category qualification.
• Add coverage for both supported seam families across status, export, benchmark snapshots, and passport-preserving rebuilds.
• Make spec generate idempotent for unchanged tracked passports by skipping timestamp-only rewrites, which keeps the ecommerce example clean on repeated checks.
• Update repo and example docs to clarify which artifacts are ephemeral versus intentionally tracked.

Testing

• cargo test -p spec-core semantic_review
• cargo test -p spec-core passport
• cargo test -p spec-cli benchmark_read_surfaces_do_not_backfill_missing_supported_seam_descriptor_ids -- --exact
• cargo test -p spec-cli spec_generate_second_run_keeps_tracked_passport_bytes_when_nothing_changed
• cargo test -p spec-cli status_benchmark_root_contract_matches_frozen_fixture -- --exact
• cargo test -p spec-cli export_benchmark_root_contract_matches_frozen_fixture -- --exact
• cargo test -p spec-cli benchmark_snapshot_writes_seeded_positive_negative_and_reserved_outputs -- --exact
• cargo fmt --all