filter: sweep the corner continuously; raise the editor floor to 840x620

Cutoff-only re-solve (15.5 vs 56.9 ns/frame) makes the unquantized sweep
affordable, replacing the 2048-step mod quantizer. Live-compute parameters
remain blocked on a shell-architecture ruling.

src/core/instrument/ui/sample_bands.h

@@ -12,6 +12,13 @@ namespace reasampler::instrument::ui {
 // Shared outer inset every band honours horizontally.
 inline constexpr int kPad = 8;
 
+// The editor's client-area floor, which IS its default size: the band stack is laid out for
+// exactly this, and there is no scroll, so anything smaller pushes the deck band off the
+// window bottom (computeSampleBands' waveform-floor-wins degrade). Growing is fine — the
+// waveform band is the elastic one. Both the enforced minimum and the opening rect read this.
+inline constexpr int kEditorMinWidth = 840;
+inline constexpr int kEditorMinHeight = 620;
+
 // Chrome band: the toolbar row (title + nav) stacked over the control row (piano strip,
 // preview, velocity knob, curve button, channel toggle). sample_chrome partitions it.
 inline constexpr int kTitleHeight = 26;