feat(creator-keys): linear vesting with cliff enforcement (#916)

[Cmitchelle7]

Closes #916

Scope

• vesting::vested_amount() — pure calculation, zero before cliff, linear release from start_timestamp over duration_secs, full allocation once elapsed >= duration.
• vesting::claimable_amount() — vested minus claimed, floored at zero.
• vesting::get_vesting_info() — aggregated view (total, vested, claimed, claimable, cliff/start/duration, is_fully_vested) in one call.
• Contract entrypoints: create_vesting_cliff, claim_vested_cliff, get_vesting_info.
• New DataKey::VestingCliffConfig(creator, beneficiary) storage key (appended per storage-key-invariants.md).
• VestingCliffClaimedEvent emitted on claim.

Design decisions

• Kept as a new, separate schedule type rather than modifying the existing VestingSchedule/create_vesting/claim_vested trio, since that feature has no cliff concept and issue Implement linear vesting release calculation with cliff enforcement #916 doesn't ask to replace it. Happy to consolidate if maintainers prefer.
• duration_secs == 0 fully vests exactly at the cliff, to avoid division by zero while still respecting the cliff gate.
• Claimed tokens are credited to the beneficiary's key balance via the existing balance/holder_count path, mirroring claim_vested's internal-ledger credit (no SEP-41 transfer in this contract).

Validation

No local Rust toolchain available (missing MSVC linker) — CI is the compile/test loop. Unit tests in vesting.rs cover pre-cliff, at-cliff, mid-vesting, fully-vested, zero-duration, zero-allocation, and rounding cases per the issue's acceptance criteria. Integration tests in vesting_cliff_lifecycle.rs cover the create/claim/info lifecycle and duplicate-schedule rejection.

[drips-wave]

@Cmitchelle7 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Chucks1093]

Conflict

[Chucks1093]

Conflict

[Chucks1093]

Conflict