vlc-plugin-ffmpeg: extend docs + runtime check

The previous vlc-plugin-dvb diagnosis was incomplete. On a laptop with
only vlc-plugin-dvb installed, VLC reads the MPEG-TS container, sees
the H.264 stream type in the PMT, then errors "Codec h264 ... is not
supported" because libavcodec_plugin.so is also a split package and
also wasn't pulled in by the base `vlc` install.

Installing vlc-plugin-ffmpeg (which pulls ffmpeg4.4 as a compat dep)
on the laptop made VLC play pixelpass cleanly via Intel iHD hardware
decode.

- README: list both plugin packages under requirements; rewrite the
  known-limitations line.
- interactive.rs: extend the launch-time check to also probe for
  libavcodec_plugin.so; combine both into one warning that lists
  every missing piece and the single pacman invocation to fix.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -67,13 +67,18 @@ pixelpass <ticket>
   - `gstreamer`, `gst-plugins-base`, `gst-plugins-good`, `gst-plugins-bad`,
     `gst-plugins-ugly`, `gst-libav`, `gst-plugin-va`, `gst-plugin-pipewire`
 - A player: `mpv` (recommended) or `vlc`
-- If you use VLC: `vlc-plugin-dvb` must also be installed. Arch / CachyOS
-  ship VLC's MPEG-TS demuxer (`libts_plugin.so`) in that separate
-  package, and without it VLC misidentifies the H.264 stream and shows
-  a green screen. mpv is unaffected.
+- If you use VLC, two split plugin packages are also needed on Arch-family
+  distros — the base `vlc` package does not pull them in:
+  - `vlc-plugin-dvb` — provides the MPEG-TS demuxer (`libts_plugin.so`).
+    Without it, VLC can't parse the container.
+  - `vlc-plugin-ffmpeg` — provides the H.264 decoder
+    (`libavcodec_plugin.so`). Without it, VLC parses the container,
+    identifies the codec as H.264, then errors with
+    `Codec h264 ... is not supported`.
+  mpv ships its own decoder stack and doesn't share either dependency.
 - PipeWire (for screencast portal + audio capture)
 
-On Arch / CachyOS:
+On Arch / CachyOS / EndeavourOS:
 
 ```sh
 sudo pacman -S gstreamer gst-plugins-base gst-plugins-good gst-plugins-bad \
@@ -81,7 +86,7 @@ sudo pacman -S gstreamer gst-plugins-base gst-plugins-good gst-plugins-bad \
                libva-utils mpv
 # plus your GPU's VAAPI driver
 # plus, if you want to use VLC instead of mpv:
-sudo pacman -S vlc vlc-plugin-dvb
+sudo pacman -S vlc vlc-plugin-dvb vlc-plugin-ffmpeg
 ```
 
 If the viewer is running on battery, set the CPU governor to performance
@@ -151,10 +156,11 @@ relay path otherwise. Both have been verified end-to-end.
 
 ## Known limitations and gotchas
 
-- **VLC needs `vlc-plugin-dvb`** to demux MPEG-TS on Arch / CachyOS. The
-  symptom of missing it is a green screen — VLC falls back to the PS
-  demuxer and misidentifies the H.264 stream. pixelpass warns at
-  player-launch time if the plugin isn't on disk. mpv doesn't need it.
+- **VLC needs `vlc-plugin-dvb` and `vlc-plugin-ffmpeg`** on Arch-family
+  distros — the base `vlc` package doesn't pull these in, and missing
+  either one breaks playback (the first kills the demuxer, the second
+  kills the H.264 decoder). pixelpass warns at player-launch time if
+  either plugin isn't on disk. mpv doesn't share these dependencies.
 - **Audio echo** if the host plays the stream through speakers and
   captures system audio — expected, the mic / monitor picks up the
   playback. Headphones bypass it.