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/ui.c

@@ -112,28 +112,8 @@ static void LoadSelectedFile(void)
     if (app.browserIsDir[app.browserSelected]) {
         NavigateToDirectory(app.browserFiles[app.browserSelected]);
     } else if (FileExists(filePath) && LoadWavFile(filePath, &app.signal)) {
-        app.loaded = true;
-        app.stftComputed = false;
-        app.loadingPhase = 0;
-        app.loadingProgress = 0.0f;
-        app.currentSTFTSegment = 0;
-        app.skipFactor = 1;
-        app.highResFinished = false;
-        app.bgHighResSeg = 0;
-        app.bgFinished = false;
-        app.isBgProcessing = false;
-        // Signal changed — free cache (results are tied to signal data)
-        FreeAllCacheEntries(&app.fftCache);
-        app.timeSelectionStart = app.viewStart = 0.0f;
-        app.timeSelectionEnd = app.viewEnd = 1.0f;
-        app.freqSelectionStart = 0.0f;
-        app.freqSelectionEnd = 1.0f;
+        ResetForNewSignal();
         app.showFileBrowser = false;
-        ComputeSTFTInit(&app.signal, &app.stft, app.fftSize);
-        // Invalidate visible texture cache
-        if (app.visibleTexture.id != 0) UnloadTexture(app.visibleTexture);
-        app.visibleTexture = (Texture2D){ 0 };
-        app.visibleTextureValid = false;
         TraceLog(LOG_INFO, "Loaded: %s", filePath);
     }
 }
@@ -382,6 +362,7 @@ void DrawSidebar(void)
     DrawSlider(dbSlider, dbValue);
     if (UpdateSlider(dbSlider, &dbValue)) {
         app.amplitudeFloorDb = -100.0f + dbValue * 80.0f;
+        app.amplitudeUserSet = true;  // stop auto-scale from overwriting this
         needsRegen = true;
     }
     y += 28 * scale;
@@ -511,7 +492,8 @@ void DrawSidebar(void)
     }
     
     if (needsRegen && app.stftComputed) {
-        GenerateSpectrogramTexture(&app.stft, &app.spectrogramImage, &app.spectrogramTexture);
+        // dB floor / colormap only — re-map the cached reassignment, don't recompute it.
+        ColorizeSpectrogram(&app.spectrogramImage, &app.spectrogramTexture);
         app.visibleTextureValid = false;  // Force cache invalidation
     }
 }