[thacer13]

Nineteen fixes, each one where Lemon Studio's state went out of step with itself: an undo that did not give the
project back, a save that did not reopen as saved, or playback that did not hold what the project says. They were found
by the equivalence harness of the control-plane plan (specs roadmap/control-plane.md part 2, step 0), which drives the
app on FL's 151 bundled projects with random clicks, drags, wheels and keys, and checks at the end of each trace that
undo goes back to the start, redo to the end, a saved copy opens as the same project and the engine holds the project.
They go in before the refactor so that its steps can each be held to "no change at all".

Each fix has a test that fails without it. The whole os-app suite passes.

Undo

• Undo and redo of a pattern edit leave the pattern shown, as in FL. Measured on FL 24.1.1: a step sequencer edit
  and a piano roll note in one pattern, undone and redone while another pattern was shown; FL kept showing the other.
  Lemon switched to the pattern edited.
• Undo of a whole-project step (adding an effect, channel and track edits) puts back the rack's loop mode and type
  with its lengths, the mixer's slot colours and icons and the rack's loop points; it leaves the pattern shown, except
  for steps that make or remove patterns (a clone, a delete), which show the pattern they showed.
• An edit no longer drops the knob step before it when the edit renumbers what the step names (a track deleted
  after a knob on a later track was wheeled: the knob's step read as unchanged and went).
• Undo of a send level puts back no level record when the route had none (it left one at unity, and the saved file
  differed).
• A piano roll tool's preview (Chop, Quantize and the others) is put back in its own pattern when another pattern is
  shown while the dialog is open; before, the previewed notes stayed, with no undo step.
• Undo steps FL makes and Lemon did not: a drag in the channel rack's graph editor ("Graph editor change velocity",
  ui/undo-steps.md), Shift+wheel moving a channel (Move channel), Open audio editor adding Tesla (measured on FL
  24.1.1: "Insert effect plugin").

Save

• The rack's loop lengths follow the pattern shown: each pattern keeps its own, as the file stores them; a save
  wrote the lengths of the pattern opened over the pattern shown.
• A saved project opens again on the pattern shown when it was saved, with that pattern's loops.
• A mixer track's equaliser is saved.
• Loop mode with every channel following the pattern survives a save.
• Stereo separation moves in 64ths, the steps the file keeps (measured on FL 24.1.1); it was continuous and only
  rounded at save.
• A new audio or automation clip holds the offsets FL writes for it (float −1.0, flp/events/playlist.md); it held
  the pattern clips' encoding until a save and reopen.

Playback

• A song with no clips plays the pattern shown, not the pattern the project was opened or saved on.
• A save hands what it writes into the project to playback (the tempo, song mode, the pattern shown, the rack's
  groups and loops, the playlist's view): the engine kept the project from before the save until the next edit.
• Tempo changes while playing reach recording and the latency box, which timed beats at the tempo of Play.
• A playlist solo is over once it no longer holds, as in the channel rack and the mixer.

Measured

The harness's recording of FL's 151 bundled projects (one 300-step trace each), on main and on this branch:

What the end checks report	Traces on main	With these fixes
a saved copy opens on another pattern than the one shown	96	0
a mixer track's equaliser not as saved	50	0
redo to the end shows another pattern than the edit left	36	0
undo to the start leaves a mixer slot filled	33	0
stereo separation not as saved	20	0
a send level after undo or save not as it was, or a channel moved with no undo step	4	0
a resave writes other loop lengths	2	0
any of these	139	0

How to check

cargo test -p os-app --bin lemonstudio (the new tests are named after what they check). The harness itself comes
in its own pull request; on this branch's build its recording of the 151 projects reports nothing that is not on its accepted list.

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