Addresses three critical network issues for LLM streaming performance:
**#33 Disable proxy buffering for SSE**
- Add X-Accel-Buffering: no header to response
- Tells nginx/Ingress to stream events immediately instead of buffering
- Paired with ResponseController.Flush() for unbuffered token delivery
**#32 HTTP/2 multiplexing for concurrent streams**
- Enable HTTP/2 in server config via http2.ConfigureServer()
- Increase MaxConnsPerHost from default (2) to 10
- ForceAttemptHTTP2 on outbound Transport for upstream connections
- Allows multiple concurrent LLM requests without blocking
**#31 TCP backpressure for streaming LLM responses**
- Set TCP_NODELAY on dialer to disable Nagle's algorithm
- Reduces latency by sending small packets immediately
- Critical for low TTFT (time-to-first-token) under load
- Upstream Transport respects backpressure when clients read slowly
**Tests added:**
- TestTCPBackpressure: Verifies TCP backpressure handling with slow client
- TestConcurrentSSEStreams: Confirms HTTP/2 multiplexing works correctly
- Both pass at 0.11s and 0.06s respectively
Fixes all three streaming performance issues in one coherent change.
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