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/spectrogram.c

@@ -53,6 +53,37 @@ static bool IsUserInteracting(void)
     return false;
 }
 
+// Reset all per-signal state after a new signal has been loaded into app.signal.
+// Drops the cached STFT/FFT-size cache and the on-screen textures so the main
+// loop recomputes from scratch (loadingPhase 0 handles the STFT (re)alloc).
+void ResetForNewSignal(void)
+{
+    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;
+    app.amplitudeUserSet = false;  // re-enable auto-scaling for the new file
+
+    // Cached STFT results are tied to the old signal data.
+    FreeAllCacheEntries(&app.fftCache);
+
+    // Reset view + selection to full range.
+    app.viewStart = 0.0f; app.viewEnd = 1.0f;
+    app.timeSelectionStart = 0.0f; app.timeSelectionEnd = 1.0f;
+    app.freqSelectionStart = 0.0f; app.freqSelectionEnd = 1.0f;
+
+    // Invalidate the cached visible texture.
+    if (app.visibleTexture.id != 0) UnloadTexture(app.visibleTexture);
+    app.visibleTexture = (Texture2D){ 0 };
+    app.visibleTextureValid = false;
+}
+
 // ============================================================================
 // Main Application
 // ============================================================================
@@ -108,7 +139,6 @@ int main(int argc, char* argv[])
     app.highResFinished = false;
     app.bgHighResSeg = 0;
     app.bgFinished = false;
-    app.lastInteractedFrame = 0;
     app.isBgProcessing = false;
     // Initialize FFT cache
     app.fftCache.count = 0;
@@ -151,19 +181,7 @@ int main(int argc, char* argv[])
 
         if (FileExists(pathToLoad) && LoadWavFile(pathToLoad, &app.signal)) {
             fileLoaded = true;
-            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);
-            ComputeSTFTInit(&app.signal, &app.stft, app.fftSize);
+            ResetForNewSignal();
             TraceLog(LOG_INFO, "File loaded successfully");
         }
     }
@@ -180,24 +198,7 @@ int main(int argc, char* argv[])
                 bool isWav = ext && (strcmp(ext, ".wav") == 0 || strcmp(ext, ".WAV") == 0 || strcmp(ext, ".Wave") == 0 || strcmp(ext, ".Wav") == 0);
                 if (isWav && FileExists(dropped.paths[0])) {
                     if (LoadWavFile(dropped.paths[0], &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.viewStart = 0.0f; app.viewEnd = 1.0f;
-                        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;
+                        ResetForNewSignal();
                     }
                 }
             }
@@ -986,6 +987,7 @@ int main(int argc, char* argv[])
     UnloadTexture(colormapTexture);
     FreeBrowserFiles();
     FreeAllCacheEntries(&app.fftCache);
+    free(app.reassignBuffer);
     FreeSignal(&app.signal);
     CloseAudioDevice();
     CloseWindow();