fix(dash): un-latch track presence so the live edge survives a mid-session track death

[ntt0601zcoder]

Problem (audit A-8, HIGH)

The DASH packager derived haveVideo/haveAudio from videoInit/audioInit, which are built once and never cleared. When one elementary stream stops mid-session — an encoder fault, or a failover swap to a single-track feed while the publisher keeps running by design — the presence flag stays latched true. buildCutDecision's V/A duration coupling then holds every cut waiting for the dead track's frames (Ok=false on every 50 ms tick), so:

• the live edge freezes permanently for all DASH viewers,
• the FrameQueue saturates maxQueueSpanMs and overflow-drops every incoming frame,
• only a manual restart recovers — and HLS on the same stream keeps working, masking the failure.

The symmetric video-death case is unreachable too: cutAudioOnly requires !haveVideo, which never becomes false while latched.

Fix

Derive track presence from arrival liveness instead of init existence:

• handleH264 / handleAAC stamp lastVideoFrameAt / lastAudioFrameAt on every accepted frame.
• New liveTrackPresence(now) declares a track dead once the stream is StateLive and that track has both an empty queue and no arrival within trackLossTimeout (6 s).
• Down-grading is gated to StateLive only — WaitingForPairing (pairing handshake) and SessionBoundary (post-reset stale timestamps) keep init-presence so neither is disturbed.

Audio death → haveAudio=false → coupling skipped (buildCutDecision's audio block already gates on haveAudio) → video cuts resume. Video death → haveVideo=false → cutAudioOnly engages. Presence flips back automatically when the track resumes (handlers re-stamp), and the wallclock-anchored tfdt lands the resumed segment at the live edge with no extra re-anchoring.

Test

TestLiveTrackPresence_TrackDeath covers: audio-dead/video-survives, video-dead/audio-survives, still-draining-queue (not dead), recent-frames (alive), waiting-for-pairing (no down-grade), and genuine single-track stream.

Package: go test -race ./internal/publisher/dash/ green, golangci-lint 0 issues, full go build ./... green.

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