Implements Issue #44 by enforcing a maximum vesting duration to prevent overflow/math edge cases.

[Edoscoba]

What changed

• Defined MAX_DURATION = 315360000 seconds (10 years).
• Reverts vault creation when (end_time - start_time) > MAX_DURATION (and rejects end_time < start_time) in:
  • create_vault_full
  • create_vault_lazy
  • batch_create_vaults_lazy
  • batch_create_vaults_full
• Added the same cap to GrantContract::initialize_grant and VestingVault::initialize for consistency across vesting entrypoints.
• Updated SPEC.md and added tests covering the cap behavior.

Testing

• cd contracts/vesting_curves && cargo test

Acceptance criteria

• Define MAX_DURATION = 315360000
• Revert create_vault if duration > MAX_DURATION

close #70

[JerryIdoko]

Resolve conflicts

[JerryIdoko]

resolve conflicts

[Edoscoba]

Done @JerryIdoko