docs(runbook): document Stellar RPC multi-provider failover and circuit breaker (#447)

[s6pa1rta3n-lab]

Summary

Resolves #447. Updates docs/RUNBOOK.md § 3 (RPC Failover) and .env.example to reflect the implemented multi-provider failover and per-endpoint circuit breaking architecture in src/stellar/client.ts.

Changes

• Architecture & Lifecycle: Replaced outdated "Option B: Multi-provider fallback (not yet implemented)" pseudocode with the actual ResilientRpcClient design, endpoint resolution hierarchy (STELLAR_RPC_URLS -> STELLAR_RPC_URL -> network default), and circuit breaker states (Closed, Open, Half-Open).
• Configuration Variables: Documented STELLAR_RPC_URLS, STELLAR_RPC_URL, and HTTP_CLIENT_* settings in docs/RUNBOOK.md and added corresponding configuration entries to .env.example.
• Operator Recovery: Documented getRpcHealthSnapshot() and resetRpcCircuitBreakers() for inspecting and resetting circuit breaker states during incidents.
• Prometheus Observability: Added documentation, curl inspection commands, and PromQL queries for stellar_rpc_attempts_total, stellar_rpc_failovers_total, stellar_rpc_circuit_open_total, and stellar_rpc_request_duration_seconds.
• Outage Playbook: Updated operational actions and endpoint health verification procedures.

Verification

• npm run env:parity passed with zero errors.
• npx jest tests/client.test.ts passed (12/12 tests).
• npx jest tests/unit/stellar/network-config.test.ts passed (10/10 tests).

Payout Routing

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