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.h

@@ -13,7 +13,11 @@ float MeasureTextScaled(const char* text, float baseSize);
 void GenerateColormapTexture(void);
 
 // --- Spectrogram texture ---
+// GenerateSpectrogramTexture recomputes the synchrosqueezed reassignment (use
+// when the STFT changes). ColorizeSpectrogram only re-maps the cached
+// reassignment to colors (use for dB-floor / colormap changes).
 void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* texture);
+void ColorizeSpectrogram(Image* image, Texture2D* texture);
 
 // --- On-screen drawing (operate on the global app state) ---
 void DrawSpectrogramGrid(Rectangle bounds, int numCellsX, int numCellsY, Color color);