fix(waveform): 4x-oversampled min/max envelope, columnMinMax homed in peaks, panel routed through shared drawWaveform — one gap-free algorithm on all surfaces

src/peaks.h

@@ -66,6 +66,21 @@ Envelope computeEnvelope(const std::vector<AudioSample>& interleaved,
                          std::size_t frameCount,
                          std::size_t binCount);
 
+// The merged min/max for display column `col` (0-based, of `columnCount` total columns)
+// of a pre-computed per-bin ChannelEnvelope: the true extremes of every bin that projects
+// to that column. This is the display-side collapse of an envelope computed at HIGHER
+// resolution than the drawn width (oversampled bins -> per-pixel-column min/max), so a
+// steep transient whose adjacent bins hold disjoint spans (e.g. {0.9,1.0} then
+// {-1.0,-0.9}) renders as one gap-free vertical span instead of two separated dots.
+//
+// Bin->column mapping mirrors computeEnvelope's half-open partition:
+//   column col owns bins [col*nbins/columnCount, (col+1)*nbins/columnCount).
+// When that range is empty (more columns than bins), the enclosing bin
+// (col*nbins/columnCount) fills the column — so no column is left empty and no bin is
+// ever dropped. columnCount <= 0 or bins.empty() returns {0, 0}; `col` is clamped to
+// [0, columnCount-1]. Pure.
+MinMax columnMinMax(const ChannelEnvelope& bins, int columnCount, int col);
+
 // Sentinel returned by lastFrameAboveThreshold when NO frame in the scanned range
 // peaks above the threshold (pure silence at that level). SIZE_MAX is unambiguous:
 // no valid frame index can equal it (a real index is < frameCount <= SIZE_MAX for