Add D5 Design-View mode switch to bank panel

New pure mode_switch module (header-rect -> N equal segment rects + point hit-test, unit-tested) plus a segmented switch in the bank_panel header: one lit segment per registered mode, click activates via view::applyMode. Grid offset below the header.

src/mode_switch.cpp

@@ -0,0 +1,64 @@
+// mode_switch — pure implementation. See mode_switch.h. NO REAPER / SWELL / vendor.
+
+#include "mode_switch.h"
+
+#include <cstddef>
+
+namespace reasampler {
+
+namespace {
+
+// The left edge of segment i in a header of the given x-origin and width divided
+// into `count` segments. Boundary i is x + (i * width) / count, so segment i spans
+// [edge(i), edge(i+1)). Because every boundary is derived from the same formula,
+// consecutive segments share an exact edge (no gap, no overlap) and edge(count)
+// == x + width precisely. count assumed >= 1 by callers.
+int segmentEdge(int x, int width, int i, int count) {
+    return x + (i * width) / count;
+}
+
+} // namespace
+
+std::vector<SegmentRect> computeSegmentRects(const HeaderRect& header,
+                                             int segmentCount) {
+    std::vector<SegmentRect> rects;
+    if (segmentCount <= 0 || header.width <= 0) return rects;
+
+    rects.reserve(static_cast<std::size_t>(segmentCount));
+    for (int i = 0; i < segmentCount; ++i) {
+        const int left = segmentEdge(header.x, header.width, i, segmentCount);
+        const int right = segmentEdge(header.x, header.width, i + 1, segmentCount);
+        SegmentRect r;
+        r.x = left;
+        r.y = header.y;
+        r.width = right - left;   // absorbs rounding; adjacent segments abut exactly
+        r.height = header.height;
+        rects.push_back(r);
+    }
+    return rects;
+}
+
+int hitTestSegment(int px, int py, const HeaderRect& header, int segmentCount) {
+    if (segmentCount <= 0 || header.width <= 0 || header.height <= 0) return -1;
+
+    // Reject anything outside the header band first (half-open bounds match the
+    // segment rects). Below the header is where the grid lives — the panel falls
+    // through to grid handling on a -1.
+    if (px < header.x || px >= header.x + header.width ||
+        py < header.y || py >= header.y + header.height)
+        return -1;
+
+    // Inside the band: find the segment whose [edge(i), edge(i+1)) contains px.
+    // Linear over N (N is tiny — one per mode); mirrors the boundary formula so the
+    // hit matches the drawn segment exactly.
+    for (int i = 0; i < segmentCount; ++i) {
+        const int left = segmentEdge(header.x, header.width, i, segmentCount);
+        const int right = segmentEdge(header.x, header.width, i + 1, segmentCount);
+        if (px >= left && px < right) return i;
+    }
+    // Guard: px == header.x + header.width would fail the < above but was already
+    // excluded by the band check. Any residual falls to -1 (defensive, unreachable).
+    return -1;
+}
+
+} // namespace reasampler