feat: add clipbin-media for reading segments with FFmpeg

One shared FFmpeg binding (ffmpeg-next 9, dynamically linked: the system
FFmpeg on Linux, the OBS-bundled DLLs on Windows) for clip export and
playback, with a probe of segment duration, codec, size and audio tracks.

crates/clipbin-media/src/lib.rs

@@ -0,0 +1,69 @@
+//! Reading recorded segments with FFmpeg: clip export (remuxing without
+//! re-encoding) and decoding for playback.
+//!
+//! FFmpeg is linked dynamically: the system's libraries on Linux (the OBS
+//! runtime already depends on them) and, on Windows, the FFmpeg DLLs shipped
+//! in the official OBS release that scripts/obs/assemble-windows.sh bundles.
+
+use std::{path::Path, sync::Once, time::Duration};
+
+pub use ffmpeg_next as ffmpeg;
+
+/// Initialises FFmpeg once per process.
+pub fn init() {
+    static INIT: Once = Once::new();
+    INIT.call_once(|| {
+        let _ = ffmpeg::init();
+        ffmpeg::log::set_level(ffmpeg::log::Level::Error);
+    });
+}
+
+/// Basic facts about a segment file.
+#[derive(Debug, Clone, PartialEq)]
+pub struct Probe {
+    pub duration: Duration,
+    pub video_codec: String,
+    pub width: u32,
+    pub height: u32,
+    pub audio_tracks: usize,
+}
+
+pub fn probe(path: &Path) -> Result<Probe, ffmpeg::Error> {
+    init();
+    let input = ffmpeg::format::input(path)?;
+    let video = input
+        .streams()
+        .best(ffmpeg::media::Type::Video)
+        .ok_or(ffmpeg::Error::StreamNotFound)?;
+    let decoder = ffmpeg::codec::context::Context::from_parameters(video.parameters())?
+        .decoder()
+        .video()?;
+    let micros = input.duration().max(0) as u64;
+    Ok(Probe {
+        duration: Duration::from_micros(micros),
+        video_codec: decoder.id().name().to_owned(),
+        width: decoder.width(),
+        height: decoder.height(),
+        audio_tracks: input
+            .streams()
+            .filter(|stream| stream.parameters().medium() == ffmpeg::media::Type::Audio)
+            .count(),
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Set CLIPBIN_TEST_SEGMENT to a recorded segment to run this.
+    #[test]
+    fn probes_a_recorded_segment() {
+        let Some(path) = std::env::var_os("CLIPBIN_TEST_SEGMENT") else {
+            return;
+        };
+        let probe = probe(Path::new(&path)).unwrap();
+        assert!(probe.duration > Duration::ZERO);
+        assert!(probe.width > 0 && probe.height > 0);
+        println!("{probe:?}");
+    }
+}