fix(S4): close graveyard epoch ordering race; add canvas-clip + stride integration tests

Tag LoadedInstrument with installedAt; process() publishes that field (not a re-read
of reloadGeneration_) so the pruner's displacedAt <= seen bound is airtight. Also fixes
sampleRowHitTest canvas.bottom over-read and adds interleave-stride + canvas-clip tests.

src/vst/editor_geometry.cpp

@@ -64,9 +64,12 @@ int sampleRowHitTest(const EditorLayout& layout, int rowCount, int x, int y) {
     // Must be within the canvas horizontally and at/below its top.
     if (x < layout.canvas.left || x >= layout.canvas.right) return -1;
     if (y < layout.canvas.top) return -1;
+    // Clip at the canvas bottom: clicks in the canvas's dead-zone below the last
+    // visible row agree with sampleRowRect, which does not clamp rows to canvas.bottom.
+    if (y >= layout.canvas.bottom) return -1;
     const int index = (y - layout.canvas.top) / kSampleRowHeight;
     if (index < 0 || index >= rowCount) return -1;
-    // Guard the bottom edge: a click below the last row's canvas bottom is outside.
+    // Guard the bottom edge: a click below the last row's bottom is outside.
     const Rect r = sampleRowRect(layout, index);
     if (y >= r.bottom) return -1;
     return index;