vlc-plugin-dvb: document and warn at launch

VLC's MPEG-TS demuxer (libts_plugin.so) ships in a separate package on
Arch / CachyOS (vlc-plugin-dvb). Without it, VLC silently falls back
to the PS demuxer and misidentifies our H.264 stream — the symptom is
a green screen. mpv doesn't share this dependency.

- README: list vlc-plugin-dvb under requirements, replace the
  "green screen, not yet diagnosed" gotcha with the diagnosis.
- interactive.rs: when the user picks VLC, check for
  /usr/lib/vlc/plugins/demux/libts_plugin.so and print a warning to
  stderr if it's missing. Soft warning, not a hard error — VLC still
  spawns so the user can confirm the symptom for themselves.

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

src/interactive.rs

@@ -68,14 +68,35 @@ impl Player {
                     url,
                 ],
             ),
-            Player::Vlc => crate::common::process::spawn_detached(
-                "vlc",
-                &["--network-caching=200", "--live-caching=200", url],
-            ),
+            Player::Vlc => {
+                warn_if_vlc_ts_demuxer_missing();
+                crate::common::process::spawn_detached(
+                    "vlc",
+                    &["--network-caching=200", "--live-caching=200", url],
+                )
+            }
         }
     }
 }
 
+// VLC's MPEG-TS demuxer ships in a separate package on Arch / CachyOS
+// (`vlc-plugin-dvb`). Without it VLC falls through to the PS demuxer
+// and shows a green screen.
+fn warn_if_vlc_ts_demuxer_missing() {
+    const TS_DEMUX_PLUGIN: &str = "/usr/lib/vlc/plugins/demux/libts_plugin.so";
+    if !std::path::Path::new(TS_DEMUX_PLUGIN).exists() {
+        eprintln!();
+        eprintln!(
+            "Warning: VLC's MPEG-TS demuxer plugin ({TS_DEMUX_PLUGIN}) is missing."
+        );
+        eprintln!(
+            "         On Arch / CachyOS, install `vlc-plugin-dvb`. Without it,"
+        );
+        eprintln!("         VLC will show a green screen. mpv is unaffected.");
+        eprintln!();
+    }
+}
+
 pub fn prompt_player() -> Result<Player> {
     let theme = ColorfulTheme::default();
     let choice = Select::with_theme(&theme)