Variable amortization schedules in lending

[nanaf6203-bit]

Problem Statement

Only level-payment supported.

Why it matters

Real loans are tailorable.

Technical Context

Add Schedule::Bullet | Annuity | Linear | Custom.

Expected Outcome

Multiple schedules.

Acceptance Criteria

1. Tests for each type.
2. SDK.
3. Docs.
4. Kani harness.

Implementation Notes

Schedule module.

Files or modules likely to be affected

contracts/lending/src/lib.rs.

Dependencies

None.

Difficulty level

HARD.

Estimated effort

L (~1 week).