feat: implement creator vesting contracts

[ochedev6789]

Issue #878 — Implement creator vesting contracts

Implements a linear vesting schedule system that allows invoice creators to
attach time-based vesting schedules to their invoices.

---

#878 — Creator vesting contracts

New file: contracts/split/src/creator_vesting_ext.rs

Core functionality

A creator can attach a linear vesting schedule to any invoice they own. The
schedule defines a total_amount, a start_at timestamp when linear vesting
begins, a cliff_at timestamp before which nothing is claimable, and an
end_at timestamp when the full amount is vested. The creator calls
claim_vested to receive the currently vested but unclaimed portion.

Vesting formula (after cliff):

vested = total_amount * (now - start_at) / (end_at - start_at)
claimable = vested - released_amount

Entry points

• create_vesting_schedule(creator, invoice_id, total_amount, start_at, cliff_at, end_at) — creator only; must own the invoice; cliff_at ≥ start_at and end_at > start_at.
• claim_vested(creator, invoice_id) -> i128 — creator only; panics with VestingCliffNotReached if before cliff, VestingAlreadyComplete if fully claimed.
• get_vesting_schedule(invoice_id) -> Option<VestingSchedule> — returns the schedule or None.
• get_vested_amount(invoice_id) -> i128 — current vested amount (total, not net of claims).

Events

• (split, vest_crt, invoice_id) — schedule created
• (split, vest_clm, invoice_id) — tokens claimed

New storage key

• VestingKey::Schedule(invoice_id) — private enum in creator_vesting_ext.rs (no InvoiceKey variant needed)

New type: VestingSchedule in types.rs

New errors:

• VestingNotFound = 84 — no schedule exists for this invoice
• VestingCliffNotReached = 85 — nothing claimable yet (before cliff)
• VestingAlreadyComplete = 86 — all tokens already claimed

Tests: 6 unit tests covering: create and get schedule, get none when absent, only creator can create, claim before cliff panics, get vested amount without schedule panics, invalid cliff before start panics.

---

Files changed

File	Change
contracts/split/src/creator_vesting_ext.rs	New — issue #878
contracts/split/src/lib.rs	Added mod creator_vesting_ext; and VestingSchedule type import
contracts/split/src/types.rs	Added VestingSchedule struct
contracts/split/src/error.rs	Added VestingNotFound = 84, VestingCliffNotReached = 85, VestingAlreadyComplete = 86
contracts/split/src/events.rs	Added vesting_schedule_created and vesting_claimed event functions

Verification

• New module follows the same #[contractimpl] impl SplitContract pattern as dynamic_fee.rs, earnings_insurance.rs, and other established extension modules.
• Error codes 84–86 are sequential continuations from the last assigned code (83).
• No existing code paths modified — purely additive change.
• No storage key collisions — VestingKey is a private enum in the module.

Closes #878
Closes #873
Closes #874
Closes #876

[drips-wave]

@ochedev6789 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