feat: convert Rockbox's playback log on the laptop, skipping the plugin

Scrobbling previously needed the on-device Last.fm plugin run by hand before
each sync, to turn Rockbox's playback log into AUDIOSCROBBLER format. Forgetting
that step means the sync submits nothing and quietly appears not to work.

Core Rockbox writes ROCKBOX_DIR/playback.log whenever "play log" is enabled,
with no plugin running at all. Each line is timestamp:elapsed_ms:length_ms:path.
The only thing missing is tags, and that is exactly why the plugin exists:
reading them back off the player is slow. Off the mirror it is free, because the
same files are already there -- so the conversion belongs on the laptop, and the
plugin can be skipped entirely.

A play counts as listened at half the track's length, matching the plugin's
savepct default, so the two cannot disagree about what a play was. A short play
is a skip. An entry with no usable timestamp is refused rather than invented,
which is the clockless case again. A path that maps to nothing in the mirror is
counted and reported instead of guessed at.

Rotated logs are picked up too; Rockbox starts a new one past half a megabyte.
All of them are renamed aside together once Last.fm has accepted the batch.

A .scrobbler.log is still read when the plugin has been run and left one.

tools/sync-to-ipod.sh

@@ -158,7 +158,11 @@ if $scrobble; then
     else
         scrobble_options=()
         $dry_run && scrobble_options+=(--dry-run)
-        python3 "$here/submit_scrobbles.py" "${scrobble_options[@]}" "$destination" ||
+        # --mirror lets it convert Rockbox's own playback.log, so the on-device
+        # scrobbler plugin never has to be run. The device root, not the music
+        # directory: the logs live in .rockbox.
+        python3 "$here/submit_scrobbles.py" "${scrobble_options[@]}" \
+            --mirror "$mirror" --device-prefix "$device_prefix" "$mounted_on" ||
             die "submitting scrobbles failed; nothing has been copied"
     fi
 fi