feat(multisig): implement EscrowEngine orchestration and module index

Chain-agnostic escrow engine routes to EVM/BTC/Solana adapters.
Manages full lifecycle: create, propose, sign, broadcast, dispute.
Feature flags: MULTISIG_ESCROW_ENABLED, MULTISIG_DEFAULT.

Engine handles:
- Adapter registry and chain routing
- Escrow persistence and event logging
- Proposal tracking and signature collection
- Threshold enforcement (2-of-3)
- Dispute flow with arbiter resolution

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ralyodio