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* feat(playback): formalize resumable direct streams and stall observability Implements #443: strong stat-based ETag + If-Range on original-file direct play (via http.ServeContent), stream-end outcome classification in RollingDeadlineWriter (stalled_reap vs client_gone vs completed) with a structured log event and Prometheus counters, the direct_stream_resume_v1 protocol-v3 capability, and a contract doc. Progressive remux is explicitly excluded from the resume contract. Code written by OpenAI Codex CLI (gpt-5.6-sol) from a Claude-authored spec; reviewed and verified by Claude. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> * fix(playback): harden direct stream resume contract * test(playback): cover resume platform contracts --------- Co-authored-by: Claude Fable 5 <noreply@anthropic.com>