Stack L/R waveform lanes in stereo mode, with overlays drawn once at full band height

src/shell/instrument/reasampler_editor.h

@@ -262,6 +262,28 @@ private:
     // thread only (file I/O); cleared with the thumbnail cache on refresh.
     const std::vector<AudioSample>& monoPcmFor(const std::string& sampleId);
 
+    // The interleaved source PCM behind the stereo waveform lanes.
+    struct ChannelPcm {
+        std::vector<AudioSample> interleaved;  // frame-interleaved source frames
+        int channelCount = 0;                  // 0 = nothing decoded
+        std::int64_t frameCount() const {
+            return channelCount > 0
+                       ? static_cast<std::int64_t>(interleaved.size()) / channelCount
+                       : 0;
+        }
+    };
+
+    // The interleaved PCM + channel count for a bank sample id. SINGLE-SLOT by design: the
+    // waveform band draws one capture at a time, while monoPcmFor's cache spans every
+    // browsed card — holding interleaved PCM there would pin a whole bank at multi-channel
+    // size. A miss re-decodes (only on selection change or a bank refresh; an edit commit
+    // does not clear it). UI thread only (file I/O).
+    const ChannelPcm& channelPcmFor(const std::string& sampleId);
+
+    // The project-relative WAV path for a bank sample id: the live bank blob first, the
+    // instance's own SampleRefs as the self-contained fallback. "" when unresolvable.
+    std::string samplePathFor(const std::string& sampleId) const;
+
     // The effective loop + start markers for the loaded capture: the parameter set's
     // override when one is set, else the bank's loop intrinsic / frame 0. Absent loop ->
     // loopStart==loopEnd==0. `frames` defaults loopEnd when the bank left the loop empty.
@@ -434,6 +456,11 @@ private:
     // Decoded mono-PCM cache, keyed by id (width-independent). Feeds the waveform envelope
     // binning + zero-crossing snap. Cleared alongside thumbCache_ on refresh.
     std::unordered_map<std::string, std::vector<AudioSample>> pcmCache_;
+
+    // The single-slot interleaved-PCM cache behind channelPcmFor (see its note on why this
+    // is not keyed into pcmCache_). Cleared alongside pcmCache_ on refresh.
+    std::string channelPcmId_;
+    ChannelPcm channelPcm_;
 };
 
 }  // namespace reasampler::vst