volume-manager: the volumes query verb — read a volume's id, path, and label (S2)

The mechanism the id/label split needs: a `volumes` verb whose reply packs the
mounted volume's {id, mount_path, label} into the tail (VolumeInfo.encode/decode
— three length-prefixed strings). Software keys on the id (the mount path is
/volumes/<id>); a shell or file manager shows the label — the database id/name
split made a query. The VM's onVolumes answers from the mounted volume, empty
reply if none. Two host round-trip tests (encode/decode; too-small buffer and
short-tail rejection). No runtime consumer yet — the first is a userspace shell;
the hello handshake is unaffected (fat-mount/volume-probe green).

system/services/volume-manager/volume-manager.zig

@@ -348,7 +348,24 @@ fn onHello(_: void, invocation: Invocation(volume_manager_protocol.Hello), _: An
     return 0;
 }
 
-const handlers = Serve.Handlers{ .hello = onHello };
+/// Answer a `volumes` query with the mounted volume's descriptor — its id (its
+/// mount path is /volumes/<id> unless overridden), its actual mount path, and
+/// its display label. This is how a shell or file manager reads a volume's
+/// friendly name: software keys on the id, a UI shows the label. An empty reply
+/// means no volume is mounted.
+fn onVolumes(_: void, _: Invocation(volume_manager_protocol.Volumes), answer: Answer(void)) isize {
+    const v = volume orelse return 0;
+    var id_buf: [volume_map.id_maximum]u8 = undefined;
+    const info = volume_manager_protocol.VolumeInfo{
+        .id = volume_map.idString(v.identity, &id_buf),
+        .mount_path = v.mount_prefix,
+        .label = v.identity.labelSlice(),
+    };
+    const encoded = info.encode(answer.tail()) orelse return 0;
+    return @intCast(encoded.len);
+}
+
+const handlers = Serve.Handlers{ .hello = onHello, .volumes = onVolumes };
 
 fn onMessage(message: []const u8, out: []u8, sender: u32, arrived: *ipc.Arrival) usize {
     // No verb takes a capability up, so the turn closes whatever arrives.