fix: harden node adapter lifecycle and performance

[smiggleworth]

Summary

• harden Node request trust boundaries, response streaming, and server lifecycle behavior
• make WebSocket and MCP shutdown and cancellation deterministic, including a bounded WebSocket close grace period
• add native Vitest benchmarks and retain measured request-body and static-asset performance improvements

Validation

• npm run fmt -- --check
• npm run check
• npm audit
• npm outdated
• npm run bench:http

Benchmark highlights

• static asset delivery improved 17-32% in repeated isolated samples
• consumed POST bodies improved about 8%
• serve() empty response handling improved about 12%

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