fix(vesting): isolate claim and clawback race handling

[tokenjunkielabs]

Fixes #513

Makes the vesting escrow claim/clawback race outcomes explicit:

• claim after a full clawback returns ClaimUnavailableAfterClawback;
• clawback after a full claim returns ClawbackUnavailableAfterFullClaim without mutating the settled grant;
• a partial claim followed by clawback preserves the already-vested remainder for the beneficiary;
• includes the three requested race-ordering regression cases.

This clean carrier supersedes closed PR #613, whose shared fork branch had unrelated scheduler changes.