feat(lib): read audio attributes from shared files at scan time

Every search reply we sent carried no bitrate or duration, so our files
failed every bitrate filter in other clients and sorted last in
SoulseekQt, whatever their quality. The share scan now reads each MP3's
first frame header (and Xing block for variable bitrate) and each FLAC's
STREAMINFO, advertising bitrate, duration, VBR, sample rate and bit
depth as far as the format gives them; other files stay bare. Only the
file's head is read, no crate is needed, and a file that does not parse
is shared without attributes rather than skipped.

soulseek-rs-lib/tests/e2e.rs

@@ -3532,3 +3532,43 @@ fn an_unanswered_upload_offer_frees_its_slot() {
 
     let _ = std::fs::remove_dir_all(share);
 }
+
+/// A constant-bitrate MP3: `frames` MPEG-1 Layer III frames at 128 kbps,
+/// 44.1 kHz, so every searcher sees a bitrate to filter and sort on.
+fn cbr_mp3(frames: usize) -> Vec<u8> {
+    let mut out = Vec::new();
+    for _ in 0..frames {
+        out.extend_from_slice(&[0xFF, 0xFB, 0x90, 0x00]);
+        out.resize(out.len() + 413, 0);
+    }
+    out
+}
+
+// A shared file with no bitrate fails every `--min-bitrate` filter and sorts
+// last in SoulseekQt; the attributes come from the file's own headers.
+#[test]
+fn a_shared_mp3_advertises_its_bitrate_and_duration() {
+    let server = server_or_skip!();
+
+    let share_dir = unique_download_dir();
+    // 1000 frames is 26 seconds.
+    std::fs::write(share_dir.join("attrprobe song.mp3"), cbr_mp3(1000))
+        .unwrap();
+    let (_sharer, searcher) = sharer_and_searcher(
+        &server,
+        &share_dir,
+        "e2e_attr_sharer",
+        "e2e_attr_searcher",
+    );
+
+    let query = "attrprobe";
+    let _ = searcher.search(query, Duration::from_secs(3));
+    let reply = reply_from(&searcher, query, "e2e_attr_sharer")
+        .expect("the sharer answers");
+    let file = &reply.files[0];
+    assert_eq!(file.attribs.get(&0), Some(&128), "bitrate in kbps");
+    assert_eq!(file.attribs.get(&1), Some(&26), "duration in seconds");
+    assert_eq!(file.attribs.get(&2), Some(&0), "constant bitrate");
+
+    let _ = std::fs::remove_dir_all(share_dir);
+}