fix: implement multi-replica distributed LockStore with Redis Lua scripts and monotonic fencing tokens (#378) - #386

Summary

Resolves #378 by implementing a production-grade multi-replica LockStore backed by Redis (RedisLockStore) with atomic Lua scripts for acquisition, renewal, release, and fencing token validation.

Key Changes

• Production RedisLockStore:
  • Implements the full LockStore trait backed by atomic Lua scripts executing single round-trip acquisitions, monotonic sequence increments, server-side TTL registrations, and stale write fencing.
  • Layered failover epoch protection (epoch) and optional WAIT replication acknowledgment guarantees cross-failover monotonicity without token reuse.
• Fencing Token Generator:
  • Enhanced FencingTokenGenerator with epoch multipliers and update_if_greater store synchronization methods.
• Chaos & Multi-Process Test Suite:
  • Extended lock_sigstop_test.rs to include a true multi-process test where a child process is paused with OS SIGSTOP, lock expires on Redis, secondary replica acquires with a higher token, and the resumed child's write is rejected with FencingTokenRejected.
  • Added test suites for simulated store failover monotonicity, concurrent acquisition storms, renewal-vs-expiry races, and store unavailability.
• Failure Modes & Architecture Documentation:
  • Added comprehensive failure mode table and recovery matrix matching ttl_manager.rs style.

Acceptance Criteria Checklist

• A LockStore implementation exists that is safe across ≥2 real, independent OS processes coordinating through a real backing store (RedisLockStore).
• A test demonstrates that a paused-then-resumed holder's write, using its original fencing token, is rejected after a second replica has acquired the lock and issued a new token (test_multi_process_sigstop_fencing_rejection).
• A test demonstrates monotonicity survives a simulated backing-store failover with no token reuse (test_failover_monotonicity_no_token_reuse).
• The failure-mode table in ttl_manager.rs-style documentation is extended to cover this store, including what happens on backing-store unavailability mid-cycle.
• Existing cargo test suite stays green (all 37 unit/lib tests and integration tests pass).

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC