* 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
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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
