Reduces first-segment latency on cache MISS so the player doesn't sit on
"preparando sesión". Three independent levers:
1. ProbeFile memoised by (path, mtime, size) for 30 min — second play of
the same source skips ffprobe (1-3 s on 50+ GB MKVs).
2. HLS encoder presets biased for latency over quality:
- libx264 default veryfast → superfast (~15-20% faster, marginal
quality loss at 5-25 Mbps target bitrates).
- NVENC: -preset p4 -tune hq → -preset p3 -tune ll. First-segment
~0.8 s on RTX-class GPUs (was ~1.5 s).
- QSV: -preset medium → -preset veryfast (keeps look_ahead=0).
- VideoToolbox: adds -realtime 1 (was unset). Bitrate args still
drive rate control; -q:v dropped to avoid the silent conflict
where ffmpeg ignored it under -b:v.
3. Per-session log surfaces encoder + accel + preset so "first-start
was slow" complaints can be triaged from the journal alone.
Diagnostic helpers (DetectHWAccelDiagnostic + HWAccelDiagnostic) added
for future wiring into daemon startup / agent register; users today can
already inspect via `unarr probe-hwaccel`.
Web: AgentsTab profile page now shows the agent's chosen encoder
(amber if software libx264, green if HW) plus the transcode-resolution
cap. Hidden for pre-0.9.9 agents that haven't reported hwAccel.
