fix(playback): wait for mpv's reply to every reused-instance IPC command (#1876)

Every reused-instance MPV IPC command now carries a request_id and waits, within the existing 2 s bound, for mpv's matching reply; an error or missing reply falls back to a fresh launch like a socket failure, and the racing post-loadfile seek is gone. MPV launches run one at a time through a launch queue so overlapping launches never act on the same child; Stop on a queued launch delegates to the in-flight launch's own closer, a failed exit stays retryable, and the dock keeps the newest launch. Position polling and output-error reporting moved out of mpv-session.service.ts, which leaves the max-lines baseline.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/playback-reused-mpv-command-replies.md

@@ -0,0 +1,9 @@
+---
+type: fix
+area: playback
+---
+
+With "Reuse player instance" on, IPTVnator now waits for MPV to confirm each
+command sent to the running player. A command MPV rejects or never answers
+falls back to a fresh MPV launch instead of reporting the stream as opened
+while nothing changed.