feat(shares): reply to browse requests with a SharedFileListResponse (code 5)

GetShareFileList (peer code 4) previously replied with a malformed message (a
server SharedFolders stats message over a peer connection). It now emits
ShareListRequested, and the client builds a real SharedFileListResponse
(code 5) from its shares grouped by directory. Adds a std-only code-5
encoder/decoder (build_shared_file_list / parse_shared_file_list) with a
roundtrip test, and Shares::directories() to group files. The decoder is the
foundation for the Browse feature.

soulseek-rs-lib/src/message/peer/shared_file_list.rs

@@ -0,0 +1,101 @@
+//! `SharedFileListResponse` (peer code 5): a peer's full shared-file listing,
+//! sent in reply to `GetShareFileList` (code 4). The payload is zlib-compressed
+//! and groups files by their virtual directory.
+
+use crate::message::Message;
+use crate::utils::zlib::{compress_stored, deflate};
+
+/// One shared directory and the files directly in it (basename + size).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SharedDirectory {
+    pub name: String,
+    pub files: Vec<(String, u64)>,
+}
+
+/// Build a `SharedFileListResponse` (peer code 5) from the directory listing.
+#[must_use]
+pub fn build_shared_file_list(dirs: &[SharedDirectory]) -> Message {
+    let mut payload = Message::new();
+    payload.write_int32(dirs.len() as u32);
+    for dir in dirs {
+        payload
+            .write_string(&dir.name)
+            .write_int32(dir.files.len() as u32);
+        for (name, size) in &dir.files {
+            payload
+                .write_int8(1)
+                .write_string(name)
+                .write_int64(*size)
+                .write_string("") // extension
+                .write_int32(0); // attribute count
+        }
+    }
+    payload.write_int32(0); // unknown
+    payload.write_int32(0); // number of private directories
+
+    let compressed = compress_stored(&payload.get_data());
+    Message::new()
+        .write_int32(5)
+        .write_raw_bytes(compressed)
+        .clone()
+}
+
+/// Parse the (zlib-compressed) `SharedFileListResponse` payload. `message` must
+/// be positioned at the compressed blob (the dispatcher sets pointer 8).
+///
+/// Returns an empty listing if the payload is malformed.
+#[must_use]
+pub fn parse_shared_file_list(message: &mut Message) -> Vec<SharedDirectory> {
+    let pointer = message.get_pointer();
+    let size = message.get_size();
+    let compressed = message.get_slice(pointer, size);
+    let Ok(data) = deflate(&compressed) else {
+        return Vec::new();
+    };
+
+    let mut body = Message::new_with_data(data);
+    let dir_count = body.read_int32();
+    let mut dirs = Vec::new();
+    for _ in 0..dir_count {
+        let name = body.read_string();
+        let file_count = body.read_int32();
+        let mut files = Vec::new();
+        for _ in 0..file_count {
+            body.read_int8(); // code
+            let filename = body.read_string();
+            let file_size = body.read_int64();
+            body.read_string(); // extension
+            let attr_count = body.read_int32();
+            for _ in 0..attr_count {
+                body.read_int32();
+                body.read_int32();
+            }
+            files.push((filename, file_size));
+        }
+        dirs.push(SharedDirectory { name, files });
+    }
+    dirs
+}
+
+#[test]
+fn shared_file_list_roundtrips() {
+    let dirs = vec![
+        SharedDirectory {
+            name: "music\\album".to_string(),
+            files: vec![
+                ("song one.flac".to_string(), 123),
+                ("song two.flac".to_string(), 456),
+            ],
+        },
+        SharedDirectory {
+            name: "music".to_string(),
+            files: vec![("top.mp3".to_string(), 789)],
+        },
+    ];
+    let message = build_shared_file_list(&dirs);
+
+    // Decode via the same offset the dispatcher would use.
+    let mut decoded = Message::new_with_data(message.get_buffer());
+    decoded.set_pointer(8);
+    assert_eq!(parse_shared_file_list(&mut decoded), dirs);
+}