Implement multi-tranche release schedule with configurable cliff period

[Kingsman-99]

Overview

Support invoices where funds are released in scheduled tranches rather than all at once. Contributors or clients can structure milestone-based payments with a cliff (nothing released until a date) followed by periodic tranches.

Requirements

• Extend InvoiceOptions with tranches: Option<Vec<Tranche>> where Tranche { release_time: u64, bps: u32 } (basis points of total)
• bps across all tranches must sum to exactly 10000 (validated at creation)
• release_tranche(env, invoice_id, tranche_index) entry point — releases one tranche if current time ≥ its release_time and invoice is fully funded
• Store released tranche indices to prevent double-release
• Emit TrancheReleased { invoice_id, tranche_index, amount } event
• Write tests: full vesting schedule, attempt release before time panics, double-release panics, bps not summing to 10000 panics at creation

Acceptance Criteria

• Tranche validation at creation (sum of bps = 10000)
• Per-tranche release with time gate
• Double-release protection
• Storage snapshot updated
• All existing tests pass
• cargo test --workspace passes with zero failures
• cargo clippy --all-targets -- -D warnings produces zero warnings

Definition of Done

All CI checks must pass before the PR is reviewed.

[lemarjohnny781]

please i would like to work on this issue

[grantfox-oss]

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