Prevent schedule arithmetic drift at maximum ledger and timestamp values #1796

Problem

Large timestamps, ledger values, or rate products can produce schedule drift or overflow near protocol limits.

Scope

Audit schedule arithmetic and use checked operations with explicit rounding rules. Document the supported numeric bounds.

Acceptance criteria

• Maximum supported values never wrap or panic.
• Rounding is consistent across create, query, and payout calculations.
• Boundary behavior is captured in property tests.

Validation

Run property-based tests over generated schedules and extreme numeric inputs.