Split the manual insertion block into two tiers so "Play later" no longer
behaves identically to "Play next":
- add a `manual_later_count` to InternalState tracking the trailing
"Play later" sub-block: [now][play-next block][play-later block][source].
- play-next inserts at the FRONT of the manual block; play-later appends at
its END (after any earlier play-later tracks), in order.
- fix the idle + shuffle case: with no current track, both actions anchor at
base 0 instead of the end of shuffle_order, so an idle-built queue plays
play-next first and play-later right after, and does not collapse the rest
of the queue on the first transport play.
- next()/remove_at()/jump decrement drain the play-next tier before the
play-later tier; set_queue/clear/shuffle-enable dissolve both.
- the public QueueState.manual_next_count exposes mnc+mlc so both tiers render
inside the single "Next in queue" visual container.
NorinB's shuffle-randomization report keeps a debug log in next() to capture
a repro if it recurs; the temporary INFO instrumentation is lowered to debug.
MixView Q-mixes reach transport parity with playlists: functional multiselect
(SelectionModel + multi-select bar), header queue menu (play next / later /
add to queue via bulkTracksAction), and create-playlist-from-selection.
Queue engine covered by 86 passing tests (play-later ordering non-shuffle,
under shuffle, and idle-shuffle cases added).
cc
vicrodh
cab68c9209