Add grace period functionality for penalty-free early exits

[BigJohn-dev]

Closes #481

Summary

Adds a configurable penalty-free grace period for near-maturity early exits in the escrow contract. When a funded commitment is refunded within the configured window before maturity, the contract now waives the early-exit penalty and returns the full principal.

Changes

• lib.rs

  • Added DataKey::GracePeriodSeconds storage key.
  • Added admin setter set_grace_period(admin, grace_period_seconds).
  • Added getter get_grace_period().
  • Implemented grace-period check in execute_refund() and dispute-resolution refunds.
  • Added missing storage keys for Paused, YieldPool, and Attestations(u64) to match contract state usage.
  • Added AttestationRecord type for attestation history storage.

• test.rs

  • Added tests covering:
    • penalty-free refund at the exact grace-window boundary
    • penalty application just outside the grace window
    • admin can set/get grace period configuration

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