feat(bank_panel): docked bank grid with LICE waveform thumbnails (M5 Wave A)

Docked SWELL window toggled by a new action, drawing the current bank as
per-sample min/max thumbnails (PCM_source + peaks) with an in-memory cache.
Pure grid-layout/cache-key math in bank_grid (tested). Renames peaks::Sample
-> AudioSample to avoid colliding with bank_model::Sample.

src/bank_grid.cpp

@@ -0,0 +1,65 @@
+// bank_grid — pure implementation. See bank_grid.h. NO REAPER / SWELL / vendor.
+
+#include "bank_grid.h"
+
+namespace reasampler {
+
+int columnsForWidth(int panelWidth, const GridSpec& spec) {
+    // Layout: [gap][cell][gap][cell]...[cell][gap]. n cells occupy
+    // gap + n*(cellWidth + gap). Solve for the largest n that fits panelWidth,
+    // clamped to at least 1 so a too-narrow panel still shows a (clipped) column.
+    const int cell = spec.cellWidth + spec.gap;
+    if (cell <= 0) return 1;  // degenerate spec — one column, avoid divide-by-zero
+    const int usable = panelWidth - spec.gap;
+    if (usable < spec.cellWidth) return 1;
+    const int cols = usable / cell;
+    return cols < 1 ? 1 : cols;
+}
+
+std::vector<CellRect> computeCellRects(int itemCount,
+                                       int panelWidth,
+                                       const GridSpec& spec) {
+    std::vector<CellRect> rects;
+    if (itemCount <= 0) return rects;
+
+    const int cols = columnsForWidth(panelWidth, spec);
+    rects.reserve(static_cast<std::size_t>(itemCount));
+
+    for (int i = 0; i < itemCount; ++i) {
+        const int col = i % cols;
+        const int row = i / cols;
+        CellRect r;
+        r.x = spec.gap + col * (spec.cellWidth + spec.gap);
+        r.y = spec.gap + row * (spec.cellHeight + spec.gap);
+        r.width = spec.cellWidth;
+        r.height = spec.cellHeight;
+        rects.push_back(r);
+    }
+    return rects;
+}
+
+int contentHeight(int itemCount, int panelWidth, const GridSpec& spec) {
+    if (itemCount <= 0) return 0;
+    const int cols = columnsForWidth(panelWidth, spec);
+    // Ceil-divide item count by columns to get the row count (partial last row
+    // still occupies a full row of height).
+    const int rows = (itemCount + cols - 1) / cols;
+    return spec.gap + rows * (spec.cellHeight + spec.gap);
+}
+
+std::string thumbnailKeyString(const ThumbnailKey& key) {
+    // Length-prefix the sampleId so a delimiter byte inside an id cannot forge a
+    // collision with a different (id, width, generation) triple.
+    std::string s;
+    s.reserve(key.sampleId.size() + 32);
+    s += std::to_string(key.sampleId.size());
+    s += ':';
+    s += key.sampleId;
+    s += '|';
+    s += std::to_string(key.width);
+    s += '|';
+    s += std::to_string(key.generation);
+    return s;
+}
+
+} // namespace reasampler