Type-enforce the waveform overlay contract, narrow waveformLanes to LaneSplit, fix laneCount/cache/path-fallback bugs

src/shell/instrument/editor_session.cpp

@@ -210,7 +210,11 @@ std::string ReaSamplerEditor::samplePathFor(const std::string& sampleId) const {
     if (!processor_) return {};
     auto banksJson = processor_->bridge().readReasamplerExtState(reasampler::kProjExtBanksKey);
     if (banksJson) {
-        if (auto sel = selectSample(*banksJson, sampleId)) return sel->relativePath;
+        // An empty relativePath (found the entry, but it carries no path) falls through to
+        // the refs fallback below rather than short-circuiting on it.
+        if (auto sel = selectSample(*banksJson, sampleId); sel && !sel->relativePath.empty()) {
+            return sel->relativePath;
+        }
     }
     // Fallback: the bank blob is not readable (extension absent / not yet parsed) or the id
     // went stale there — the instance-owned ref still carries the path, so a self-contained
@@ -222,7 +226,7 @@ std::string ReaSamplerEditor::samplePathFor(const std::string& sampleId) const {
 
 const ReaSamplerEditor::ChannelPcm& ReaSamplerEditor::channelPcmFor(
     const std::string& sampleId) {
-    if (channelPcmId_ == sampleId && !sampleId.empty()) return channelPcm_;
+    if (channelPcmId_ == sampleId) return channelPcm_;
 
     // A failed decode is still cached (channelCount stays 0) so a broken/missing file is not
     // re-read on every paint.