rekor: use checkpoint_key_id when available

[ECD5A]

Summary

Closes #1364.

Rekor checkpoint verification previously discarded most TransparencyLogInstance metadata and selected a verifier using the bundle entry's log_id. That does not work for trust roots whose signed-note checkpoint key ID differs from the log ID.

This change:

• retains each valid Rekor TransparencyLogInstance in the trusted keyring;
• indexes checkpoint verifiers by signed-note origin and checkpoint_key_id;
• falls back to the first four bytes of log_id for older trust roots without checkpoint_key_id;
• keeps inclusion-promise verification keyed by the complete log ID; and
• adds regression coverage for distinct checkpoint/log IDs, origin mismatches, and trusted-root metadata propagation.

Validation:

• pytest: 197 passed, 30 skipped, 2 xpassed
• ruff check .
• ruff format --check .
• mypy sigstore
• bandit -c pyproject.toml -r sigstore

Release Note

Used Rekor checkpoint_key_id values when verifying signed checkpoints, with a compatibility fallback for older trust roots.

Documentation

No documentation update is required. This corrects internal trusted-root key selection and preserves the existing public API.