peaks: pure per-channel min/max envelope from interleaved PCM

New STATIC lib + peaks_tests (CTest), mirroring bank_model. Float samples,
per-channel (no fold); exact integer bin spans handle remainder, short
buffers, and degenerate inputs with no OOB. Bin-span math guarded against
size_t overflow for pathological binCount.

src/peaks.h

@@ -0,0 +1,64 @@
+#pragma once
+// peaks — waveform min/max envelope (thumbnail) computation from raw interleaved
+// PCM. We compute our own thumbnails from the captured file rather than depending
+// on REAPER's peak API: we own the file format, so this is simpler, testable, and
+// dependency-free. A future bank panel (M5) calls this at whatever bin resolution
+// the panel width dictates and draws one min/max envelope per channel.
+//
+// PURE MODULE (CLAUDE.md §load-bearing split): NO REAPER types, NO SWELL, NO
+// vendor/ includes. Standard library only. Builds and unit-tests without REAPER.
+
+#include <cstddef>
+#include <vector>
+
+namespace reasampler {
+
+// Canonical in-memory sample type. `float` is REAPER's native audio buffer format
+// (its render/PCM_source callbacks hand back interleaved 32-bit float), so peaks
+// consumes that directly with no lossy conversion. If a capture ever lands as a
+// different depth, the caller converts to float at the boundary — the thumbnail
+// core stays single-typed.
+using Sample = float;
+
+// One bin of a channel's envelope: the extremes of every sample that fell in it.
+// min <= max always. For an empty bin (more bins than frames), both are 0.
+struct MinMax {
+    Sample min = 0.0f;
+    Sample max = 0.0f;
+
+    bool operator==(const MinMax& o) const { return min == o.min && max == o.max; }
+};
+
+// One channel's envelope: exactly `binCount` bins, in time order.
+using ChannelEnvelope = std::vector<MinMax>;
+
+// Per-channel envelopes: outer index is channel (channelCount entries, order
+// preserved — never mixed or folded), inner is that channel's bins.
+using Envelope = std::vector<ChannelEnvelope>;
+
+// Computes a per-channel min/max envelope from interleaved PCM.
+//
+//   interleaved  frame-interleaved samples: [f0c0, f0c1, ..., f1c0, f1c1, ...].
+//                Size must be >= frameCount * channelCount; extra is ignored.
+//   channelCount channels per frame (the stride). Each channel is enveloped
+//                INDEPENDENTLY — no averaging, no stereo fold (precision
+//                invariant: channel count preserved).
+//   frameCount   frames (samples-per-channel) to consider.
+//   binCount     requested bins per channel. Honored exactly for any frameCount.
+//
+// Frame->bin partition: frames are split into `binCount` contiguous spans as
+// evenly as possible; when frameCount does not divide evenly, the remainder is
+// spread one-frame-per-bin across the earliest bins (ceil/floor split), so the
+// tail is never dropped and no bin reads out of bounds. When binCount > frameCount
+// the trailing empty bins are {0, 0}.
+//
+// Defined behavior for degenerate input (no UB, no throw):
+//   binCount == 0     -> per channel: an empty bin vector.
+//   channelCount == 0 -> an empty envelope (no channels).
+//   frameCount == 0   -> per channel: binCount bins, all {0, 0}.
+Envelope computeEnvelope(const std::vector<Sample>& interleaved,
+                         std::size_t channelCount,
+                         std::size_t frameCount,
+                         std::size_t binCount);
+
+} // namespace reasampler