perf+ux: cache reassignment, keep manual dB floor, dedupe load paths

- Split GenerateSpectrogramTexture into ComputeSpectrogramReassignment
  (the expensive synchrosqueezing, cached in app.reassignBuffer) and
  ColorizeSpectrogram (cheap). dB-floor and colormap changes now only
  re-colorize instead of recomputing the whole reassignment every frame —
  the dB slider and colormap switching are smooth on large files.
- AutoScaleAmplitude no longer overwrites a dB floor the user set by hand
  (amplitudeUserSet flag, reset per file load).
- Extract ResetForNewSignal() used by all three load paths; removes the
  duplicated reset blocks and the double ComputeSTFTInit per load. Drag-drop
  now resets the selection like the browser already did.
- Remove the dead lastInteractedFrame field.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/render.c

@@ -96,7 +96,12 @@ void GenerateColormapTexture(void)
 }
 
 // ===== Spectrogram texture =====
-void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* texture)
+// ===== SYNCHROSQUEEZING (energy reassignment) =====
+// The expensive part: reassign energy to true frequencies using the derivative
+// STFT. Depends only on the STFT data, so the result is cached in
+// app.reassignBuffer and reused by ColorizeSpectrogram across dB-floor /
+// colormap changes (which don't need to recompute any of this).
+static void ComputeSpectrogramReassignment(StftResult* stft)
 {
     if (stft->numSegments == 0) return;
     int width = stft->numSegments;
@@ -104,9 +109,12 @@ void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* textu
     int fftSize = (height - 1) * 2;
     float freqPerBin = (float)stft->sampleRate / fftSize;
 
-    UnloadImage(*image);  // release previous image (NULL-safe on first call)
-    *image = GenImageColor(width, height, BLACK);
-    Color* pixels = (Color*)image->data;
+    // (Re)allocate the cached accumulation buffer for reassigned energy.
+    free(app.reassignBuffer);
+    app.reassignBuffer = (float*)calloc(width * height, sizeof(float));
+    app.reassignWidth = width;
+    app.reassignHeight = height;
+    float* accumBuffer = app.reassignBuffer;
 
     // Find max amplitude for normalization (skip NULL segments)
     float maxAmplitude = 0.0001f;
@@ -117,15 +125,9 @@ void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* textu
                 maxAmplitude = stft->segments[seg].spectrum[bin].amplitude;
     }
 
-    // ===== SYNCHROSQUEEZING =====
-    // Reassign energy to true frequencies using derivative STFT
-    
-    // Accumulation buffer for reassigned energy
-    float* accumBuffer = (float*)calloc(width * height, sizeof(float));
-    
     // Noise threshold: only reassign bins with significant energy
     float noiseThreshold = maxAmplitude * 0.01f;  // 1% of max amplitude
-    
+
     for (int seg = 0; seg < width; seg++) {
         // Skip segments that haven't been computed yet (overview/high-res transition)
         if (stft->segments[seg].spectrum == NULL) continue;
@@ -182,8 +184,22 @@ void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* textu
             accumBuffer[idx1] += amplitude * frac;
         }
     }
-    
-    // Convert accumulation buffer to colors
+}
+
+// Map the cached reassignment buffer to colors using the current dB floor/
+// ceiling and colormap. Cheap — safe to call every frame the dB slider moves
+// or when the colormap changes (no synchrosqueezing recompute).
+void ColorizeSpectrogram(Image* image, Texture2D* texture)
+{
+    if (app.reassignBuffer == NULL) return;
+    int width = app.reassignWidth;
+    int height = app.reassignHeight;
+    float* accumBuffer = app.reassignBuffer;
+
+    UnloadImage(*image);  // release previous image (NULL-safe on first call)
+    *image = GenImageColor(width, height, BLACK);
+    Color* pixels = (Color*)image->data;
+
     for (int i = 0; i < width * height; i++) {
         if (accumBuffer[i] > 0.0001f) {
             float db = AmplitudeToDecibels(accumBuffer[i]);
@@ -192,14 +208,20 @@ void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* textu
             pixels[i] = GetColormapColor(normalized, app.colormap);
         }
     }
-    
-    free(accumBuffer);
 
     if (texture->id != 0) UnloadTexture(*texture);
     *texture = LoadTextureFromImage(*image);
     SetTextureFilter(*texture, TEXTURE_FILTER_BILINEAR);
 }
 
+// Recompute the reassignment (STFT changed) and rebuild the texture.
+void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* texture)
+{
+    if (stft->numSegments == 0) return;
+    ComputeSpectrogramReassignment(stft);
+    ColorizeSpectrogram(image, texture);
+}
+
 // Compute auto-adjusted amplitude floor/ceiling from STFT data
 
 // ===== Grid, labels, selection, playhead =====