fix: restore waveform symmetry about the midline, cut deck_values' link to the bank model, and unit-test the column arithmetic

The waveform column's vertical extents move to pure component_geometry so the shared
primitive stops being untested; PlaySeconds hoists into a header-only play_seconds target.

src/shell/panel/CLAUDE.md

@@ -56,7 +56,7 @@ live in `shell/bank_ops`, a sibling directory, not here.
   - `panel_thumbnails` — the PCM→envelope thumbnail cache + the bank-change fingerprint pass.
   - `panel_audition` — the preview-playback engine.
   - `panel_bank_ops` — the menu/prompt UX skin over the promptless `shell/bank_ops` verbs.
-- `draw_kit` — shared LICE draw shell: `fillSurface`, `drawButton`/`drawSlider`/`drawListRow`/`drawWaveform`, cached-font `text()`, full interaction-state model, double-buffer preserved. Consumes `theme` + `component_geometry`.
+- `draw_kit` — shared LICE draw shell: `fillSurface`, `drawButton`/`drawSlider`/`drawListRow`/`drawWaveform`, cached-font `text()`, full interaction-state model, double-buffer preserved. Consumes `theme` + `component_geometry`. Its antialiasing contract — which surfaces alias, which cannot, and the LICE primitive each answers with — is the disposition table in `docs/product/visual-design-language.md` §8, which governs `drawWaveform` as much as this doc does. `drawWaveform`'s per-column vertical arithmetic is NOT here: it is the pure, unit-tested `component_geometry::waveformBand`/`waveformColumnSpan` pair, because this TU is DAW-verified and not unit-tested.
 
 ## Gotchas
 

src/shell/panel/draw_kit.cpp

@@ -27,6 +27,10 @@ using ui::compressAmplitudeForDisplay;
 using ui::roleColor;
 using ui::roleColorState;
 using ui::spectralColor;
+using ui::WaveformBand;
+using ui::waveformBand;
+using ui::WaveformColumnSpan;
+using ui::waveformColumnSpan;
 
 // The one place a pure KitColor becomes a LICE_pixel (LICE_RGBA(r,g,b,a), verified against
 // lice.h). The theme owns the color; the shell owns the packing.
@@ -289,11 +293,9 @@ void drawWaveform(LICE_IBitmap* bmp, const KitBox& box, const Envelope& env) {
 
     for (int ch = 0; ch < channels; ++ch) {
         const ChannelEnvelope& bins = env[static_cast<std::size_t>(ch)];
-        const int bandTop = box.y + ch * bandH;
-        const int midY = bandTop + bandH / 2;
-        const double halfSpan = (bandH / 2) - 2;
+        const WaveformBand band = waveformBand(box.y + ch * bandH, bandH);
 
-        LICE_Line(bmp, box.x + 2, midY, box.x + box.width - 2, midY,
+        LICE_Line(bmp, box.x + 2, band.midY, box.x + box.width - 2, band.midY,
                   midCol, 1.0f, 0, false);
 
         if (bins.empty() || innerW <= 0) continue;
@@ -303,28 +305,22 @@ void drawWaveform(LICE_IBitmap* bmp, const KitBox& box, const Envelope& env) {
         // alone leaves the outline stepped — a vertical span has no aa to apply — and where two
         // adjacent columns differ sharply it reads as a comb rather than one envelope. The
         // stroke is the SAME ink as the fill it edges, so it can only soften the boundary.
-        double prevTop = 0.0;
-        double prevBottom = 0.0;
+        WaveformColumnSpan prev;
         for (int col = 0; col < innerW; ++col) {
             const MinMax mm = columnMinMax(bins, innerW, col);
             const int x = box.x + 2 + col;
-            const double topF = midY - compressAmplitudeForDisplay(mm.max) * halfSpan;
-            const double bottomF = midY - compressAmplitudeForDisplay(mm.min) * halfSpan;
-            int yMax = static_cast<int>(topF);
-            int yMin = static_cast<int>(bottomF);
-            if (yMax < bandTop) yMax = bandTop;
-            if (yMin > bandTop + bandH - 1) yMin = bandTop + bandH - 1;
-            LICE_Line(bmp, x, yMin, x, yMax, waveCol, 1.0f, 0, false);
+            const WaveformColumnSpan s = waveformColumnSpan(
+                band, compressAmplitudeForDisplay(mm.max), compressAmplitudeForDisplay(mm.min));
+            LICE_Line(bmp, x, s.bottom, x, s.top, waveCol, 1.0f, 0, false);
             if (col > 0) {
                 const float xPrev = static_cast<float>(x - 1);
                 const float xF = static_cast<float>(x);
-                LICE_FLine(bmp, xPrev, static_cast<float>(prevTop), xF,
-                           static_cast<float>(topF), waveCol, 1.0f, 0, true);
-                LICE_FLine(bmp, xPrev, static_cast<float>(prevBottom), xF,
-                           static_cast<float>(bottomF), waveCol, 1.0f, 0, true);
+                LICE_FLine(bmp, xPrev, static_cast<float>(prev.topF), xF,
+                           static_cast<float>(s.topF), waveCol, 1.0f, 0, true);
+                LICE_FLine(bmp, xPrev, static_cast<float>(prev.bottomF), xF,
+                           static_cast<float>(s.bottomF), waveCol, 1.0f, 0, true);
             }
-            prevTop = topF;
-            prevBottom = bottomF;
+            prev = s;
         }
     }
 }