feat(vst): stand up VST3 instrument spike (S1) — skeleton, IPlugView↔LICE editor, REAPER bridge read

Vendor Steinberg VST3 SDK (v3.7.9_build_61); add reasampler_vst.vst3 as a second, additive build artifact with pure editor-geometry + bridge-marshal helpers under CTest.

src/vst/editor_geometry.cpp

@@ -0,0 +1,56 @@
+// editor_geometry.cpp — see editor_geometry.h. Pure math; no host types.
+
+#include "editor_geometry.h"
+
+#include <algorithm>
+
+namespace reasampler::vst {
+
+namespace {
+
+// Spike editor layout constants. These are the editor's fixed metrics; the real
+// editor (S4/S5) will parameterize as its content demands.
+constexpr int kTitleBarHeight = 28;
+constexpr int kButtonMargin = 10;
+constexpr int kButtonWidth = 120;
+constexpr int kButtonHeight = 24;
+
+}  // namespace
+
+bool contains(const Rect& r, int x, int y) {
+    if (r.width() <= 0 || r.height() <= 0) return false;
+    return x >= r.left && x < r.right && y >= r.top && y < r.bottom;
+}
+
+EditorLayout layoutEditor(int w, int h) {
+    // Clamp the surface to non-negative extents so a degenerate view can't produce
+    // inverted rects.
+    const int cw = std::max(0, w);
+    const int ch = std::max(0, h);
+
+    EditorLayout out;
+
+    // Title bar spans the top, clamped so it never exceeds the client height.
+    const int titleH = std::min(kTitleBarHeight, ch);
+    out.titleBar = Rect{0, 0, cw, titleH};
+
+    // Canvas is everything below the title bar.
+    out.canvas = Rect{0, titleH, cw, ch};
+
+    // Button sits at the top-left of the canvas, inset by a margin, and is clamped to
+    // fit inside the canvas so it never overhangs on a small view.
+    const int bx = out.canvas.left + kButtonMargin;
+    const int by = out.canvas.top + kButtonMargin;
+    const int bRight = std::min(bx + kButtonWidth, out.canvas.right);
+    const int bBottom = std::min(by + kButtonHeight, out.canvas.bottom);
+    out.button = Rect{bx, by, std::max(bx, bRight), std::max(by, bBottom)};
+
+    return out;
+}
+
+HitTarget hitTest(const EditorLayout& layout, int x, int y) {
+    if (contains(layout.button, x, y)) return HitTarget::kButton;
+    return HitTarget::kNone;
+}
+
+}  // namespace reasampler::vst