[beneaze]

Stem curves had no audible effect

When a stem separation from the library queue finished, the app reloaded the track's analysis, so the transition editor said the stems were there. But it kept the engine's cached copy of the track, which had been loaded without stems. Mix playback kept using that copy and only imitated the stem curves with volume and EQ.

• TrackUpdated now drops the cached track, so the next load picks up the stems. It also redraws the open mix's timeline: a re-analysed grid moves beats and the tempo lane, which explains a flat 120 BPM lane seen after launch.
• A stem file whose length doesn't fit the audio is now logged instead of dropped silently.
• New engine test transition_stem_lanes_mute_each_stem_on_both_sides.

Before the fix, I checked the engine on a real imported mix (Selected Deep Summer Copy, transition 1). Flattening the stem curves changes the rendered output by about 0.25–0.30 RMS, so the engine itself was fine.

Transition editor details adopted from DJ.Studio

Sources: screenshots of DJ.Studio in use and its public help pages, as docs/clean-room.md allows. The toggle placement is already documented in docs/interop/djstudio.md §5.4. No new features; DJ.Studio's risers, loop effects, side-chain and per-column solo were left out.

• Stem rows: the stem's colour fills the area under its gain curve. The waveform stays full size, grey over hatching where the stem is cut.
• Stem toggles: they now sit outside the window, the outgoing track's at its end and the incoming track's at its start.
• Equalizer category: holds Volume, Low, Mid and High; the separate Volume category is gone. Every column has a Manual entry, selected when a curve is hand-drawn; clicking it switches the timeline to drawing that curve. Outside the stems, the no-change mode is now called None.
• Colours: column headers use the full stem or band colour, and the chosen mode is filled and outlined in it.
• Lanes: the lane knobs show their value (dB / filter), and each track's unplayed audio (up to 16 s) is drawn faded beside its clip.
• Auto-open: when the playhead enters a transition, by playing or seeking, that transition is selected and its settings open. This happens only on entering, and not while a mouse button is held.
• Panel height: the Transition panel's minimum height goes from 130 to 230, so six entries and the no-stems warning fit.
• Devctl: new tcat <category> command, used for the screenshots.

Testing

• cargo clippy --workspace --all-targets -- -D warnings: clean
• cargo test --workspace: passes
• Checked in the running app with screenshots (devctl): stem rows, toggles, Equalizer columns, colours, and the auto-open on seeking into a transition (master volume at 0).

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