feat: use fast-webstreams for server rendering
Replace global WebStream constructors (ReadableStream, WritableStream,
TransformStream) with faster Node.js-backed implementations from
experimental-fast-webstreams. The patching happens synchronously in
node-environment-baseline.ts before any rendering code loads.
Includes vendored fixes:
- fix(pipe-to): drain buffered flush chunks before shutdown in specPipeTo
- perf(pipe-to): batch sync reads to reduce microtask yields
Benchmark (force-dynamic page, 4KB response):
Single client: 611 -> 1,011 req/s (+65% vs native WebStreams)
Under load: 1,167 -> 1,128 req/s (on par, -22% p99 latency)
