Honor grace periods in commitment violation checks

[dangelo352]

Fixes #498

Summary

• enforce grace_period_days in check_violations and get_violation_details
• relax max-loss during the initial grace window and duration until expires_at + grace_period_days
• use TimeUtils::checked_days_to_seconds plus checked deadline addition so overflow rejects instead of wrapping
• document grace semantics and backward-compatible tuple behavior in docs/commitment_core/SEMANTICS.md

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-core-grace-target cargo test -p commitment_core check_violations -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-core-grace-target cargo test -p commitment_core get_violation_details -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-core-grace-target cargo test -p commitment_core update_value -- --nocapture
• git diff --check