bridge-monitor: add Squid, Socket, Mayan + bench 002/003 specs

[Flotapponnier]

Adds 3 new bridges to the harness and both bench YAMLs.

Squid Router (Axelar-powered)

• POST https://apiplus.squidrouter.com/v2/route, public integrator ID
• EVM-only: Base→Arb corridor. ~330ms at p50 (fastest EVM tested)
• Solana source returns server-side 500 — TestRoute returns early, no fake failures

Socket Protocol (Bungee aggregator)

• GET https://api.socket.tech/v2/quote, public demo key
• EVM-only: Base→Arb. Routes through Across/Stargate/CCTP, returns best output
• Useful as "aggregator overhead" comparison vs. going direct

Mayan Finance (Wormhole-based intent layer)

• GET https://price-api.mayan.finance/v3/quote, keyless
• Solana-EVM only: Sol→Base, Arb→Sol. Skips pure EVM corridors
• May return ROUTE_NOT_FOUND on some corridors (API instability) — recorded as quote_failed, not latency noise

Both bench YAMLs updated: bridge-quote-latency.yml (bench 002) and bridge-fee.yml (bench 003).
Go build clean.