feat: implement escrow state machine

[henrypeters]

Added docs/match-lifecycle.md with a complete visual and reference guide for the escrow contract's match state machine, grounded in the actual contract source (contracts/escrow/src/lib.rs).

What's included:

• ASCII state machine diagram showing all transitions: Pending → Active → Completed and Pending → Cancelled (via cancel_match and expire_match)
• Mermaid diagram for rendered GitHub/docs previews
• Per-transition guard tables listing every authorization check, precondition, and the exact error code returned on violation
• Error path recovery table mapping common mistakes to their root cause and fix
• Timing reference: MIN (1 day / 17,280 ledgers), DEFAULT (30 days / 518,400 ledgers), MAX (90 days / 1,555,200 ledgers) with a ledger timeline illustration
• Events emitted at each transition and balance snapshot reasons
• Cross-references to error-codes.md, architecture.md, and oracle.md

docs/architecture.md updated to link to the new document from its Match Lifecycle section. README.md updated to include the state machine in the Documentation index.

closes #742

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