- Move TTFT timer start before .send().await so it includes server
  processing time (standard TTFT semantics)
- Add done flag to break outer while loop when stream ends
- Rewrite SseLineBuffer to use Vec<u8> with 1MB cap, fixing UTF-8
  corruption on multi-byte chars split across chunks
- Add sliding_window_schedule probes to Diagnose mode so the
  SlidingWindow inference branch is no longer dead code
- Move build_rpm_mode_plan after AppConfig::load to eliminate
  window-boundary timing drift

Support streaming responses for both OpenAI and Anthropic protocols.
Adds --stream CLI flag, provider config stream field, shared SSE line
buffer, and TTFT (time to first token) percentile tracking alongside
existing TTLT latency metrics.