feat: add deterministic refund penalty invariant coverage

[DeePrincipal-dev-lang]

Summary

Adds checked refund math and deterministic property-based coverage for escrow refund penalties.

What changed

• Hardened early-exit refund math in contracts/escrow/src/lib.rs so penalties are computed as floor(amount * penalty_bps / 10_000) with checked arithmetic and explicit guards.
• Reused the shared refund split helper for both direct refunds and dispute resolution.
• Added seeded deterministic property tests in contracts/escrow/src/test.rs covering:
  • refund + penalty == amount
  • non-negative refund values
  • penalty never exceeding principal
  • penalty_bps = 0 (full refund)
  • penalty_bps = 10_000 (zero refund)
  • overflow rejection for extreme principals
  • small amount edge cases
  • randomized mid-range values using deterministic proptest input generation
• Documented the refund math model and invariants in contracts/README.md.
• Added proptest support and refreshed the contract lockfile.

Invariants covered

• Principal is fully partitioned between refund and penalty.
• Penalties stay within bounds of the principal.
• Boundary behavior at 0% and 100% penalty is preserved.
• Overflowed intermediate multiplication is rejected instead of wrapping.

Verification

• cargo test --quiet in contracts/escrow passes.
• Result: 15 passed, 0 failed.

Notes

• Branch pushed: feature/refund-penalty-property-tests

Closes #471

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