fix(vesting): handle claim and clawback race states

[tokenjunkielabs]

Fixes #513

Makes the vesting escrow race outcomes explicit instead of allowing opaque or silent terminal behavior:

• 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 still leaves the already-vested remainder claimable;
• adds focused regression cases for all three ordering scenarios.

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