S6: embedded TCP/MCP UI via IReaperUIEmbedInterface

Inline keymap/level strip drawn into REAPER's embed bitmap, reusing the LICE
idiom. Pure embed_strip layout/hit-test (tested); processor exposes the embed
interface off queryInterface and publishes an RT-safe block peak for the level.

src/vst/embed_strip.cpp

@@ -0,0 +1,86 @@
+// embed_strip.cpp — see embed_strip.h. Pure math; no host types.
+
+#include "embed_strip.h"
+
+#include <algorithm>
+
+namespace reasampler::vst {
+
+namespace {
+
+// Clamp a MIDI note to [0, kEmbedKeyCount-1].
+int clampNote(int n) {
+    if (n < 0) return 0;
+    if (n > kEmbedKeyCount - 1) return kEmbedKeyCount - 1;
+    return n;
+}
+
+// Map a key boundary in [0, kEmbedKeyCount] to an x pixel inside a band of the given
+// left/width. keyEdge is a boundary (0..128), so keyEdge==128 maps to the band's right.
+// Integer math, floored — a zone's left uses floor(low) and its right uses floor(high+1),
+// which tiles adjacent zones without a seam.
+int keyEdgeToX(int bandLeft, int bandWidth, int keyEdge) {
+    if (keyEdge <= 0) return bandLeft;
+    if (keyEdge >= kEmbedKeyCount) return bandLeft + bandWidth;
+    return bandLeft + (keyEdge * bandWidth) / kEmbedKeyCount;
+}
+
+}  // namespace
+
+EmbedLayout layoutEmbed(int w, int h) {
+    const int cw = std::max(0, w);
+    const int ch = std::max(0, h);
+
+    EmbedLayout out;
+
+    // The level band takes a fixed height at the bottom, but never so much that the keymap
+    // above it falls below its minimum (or that the band exceeds the area). On a very short
+    // area the band yields to the keymap entirely.
+    int bandH = std::min(kEmbedLevelBandHeight, ch);
+    if (ch - bandH < kEmbedKeymapMinHeight) {
+        bandH = std::max(0, ch - kEmbedKeymapMinHeight);
+    }
+    const int keymapBottom = ch - bandH;
+
+    out.keymap = Rect{0, 0, cw, keymapBottom};
+    out.levelBand = Rect{0, keymapBottom, cw, ch};
+    return out;
+}
+
+Rect zoneSegmentRect(const EmbedLayout& layout, int lowNote, int highNote) {
+    const Rect& band = layout.keymap;
+    const int bandWidth = std::max(0, band.width());
+
+    int lo = clampNote(lowNote);
+    int hi = clampNote(highNote);
+    if (lo > hi) lo = hi;  // defensive: a malformed zone collapses rather than inverts
+
+    const int leftX = keyEdgeToX(band.left, bandWidth, lo);
+    const int rightX = keyEdgeToX(band.left, bandWidth, hi + 1);
+    return Rect{leftX, band.top, std::max(leftX, rightX), band.bottom};
+}
+
+int zoneAtPoint(const EmbedLayout& layout, const EmbedZone* zones, int zoneCount, int x,
+                int y) {
+    if (zoneCount <= 0 || zones == nullptr) return -1;
+    if (!contains(layout.keymap, x, y)) return -1;
+    // First covering zone in draw order wins (first-match, mirroring the core's resolve).
+    for (int i = 0; i < zoneCount; ++i) {
+        const Rect r = zoneSegmentRect(layout, zones[i].lowNote, zones[i].highNote);
+        if (contains(r, x, y)) return i;
+    }
+    return -1;  // on the band but on an uncovered key
+}
+
+Rect levelFillRect(const EmbedLayout& layout, double level) {
+    const Rect& band = layout.levelBand;
+    if (band.width() <= 0 || band.height() <= 0) return Rect{};
+    double l = level;
+    if (l < 0.0) l = 0.0;
+    if (l > 1.0) l = 1.0;
+    const int fillW = static_cast<int>(l * band.width());
+    if (fillW <= 0) return Rect{};
+    return Rect{band.left, band.top, band.left + fillW, band.bottom};
+}
+
+}  // namespace reasampler::vst