perf: index core commitments by creation bucket

[dangelo352]

Summary

• add UTC-day creation buckets for commitment_core range queries
• update create_commitment to populate the bucket index while preserving AllCommitmentIds
• make get_commitments_created_between read indexed buckets instead of scanning every commitment
• add boundary, empty/reversed range, multi-bucket, and full-scan equivalence coverage
• record the before/after cost shape in benchmarks/results

Fixes #497.

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-core-bucket-target cargo test -p commitment_core test_create_commitment_updates_storage_layout -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-core-bucket-target cargo test -p commitment_core get_commitments_created_between -- --nocapture
• git diff --check