bench 208: perp DEX liquidation rate

[Flotapponnier]

Summary

• Adds bench 208 spec (benchmarks/perp-liq-rate.yml): 24h liquidation rate (liq_usd / OI_usd * 100) across 7 perp DEX venues for ETH and BTC
• Adds full Go harness (harnesses/perp-liq-rate/) with Prometheus exporter, 24h sliding window, per-venue OI fetchers

Venues covered

• Hyperliquid (liquidations via 0xArchive + vault fallback, OI via info API)
• dYdX v4 (liquidations via REST fills, OI via markets endpoint)
• GMX Arbitrum (OI via /markets/info 30-decimal notional, liquidation source TBD)
• Lighter (liquidations via Coinalyze hourly buckets, OI via REST)
• Gains.trade Base (liquidations via Arbitrum RPC eth_getLogs, OI via trading-variables)
• Aevo (liquidations via trade fills, OI via REST)
• Paradex (liquidations via fills, OI via markets)

Apple-to-apple definition

All venues use notional USD (collateral x leverage at entry price) for both OI and liquidation events — no TVL proxies.

Test coverage

Unit + integration tests for all 7 sources. All pass with go test ./....