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_types.h

@@ -139,10 +139,17 @@ typedef struct {
     // Display settings
     float amplitudeFloorDb;
     float amplitudeCeilingDb;
+    bool amplitudeUserSet;  // true once the user adjusts the dB floor; suppresses auto-scale
     ColormapType colormap;
     bool showGrid;
     int fftSize;  // Current FFT size (128-2048)
 
+    // Cached synchrosqueezed energy (the expensive reassignment result).
+    // Reused across dB-floor / colormap changes — only re-colorized, not recomputed.
+    float* reassignBuffer;
+    int reassignWidth;
+    int reassignHeight;
+
     // File browser state
     bool showFileBrowser;
     char browserPath[512];
@@ -174,7 +181,6 @@ typedef struct {
     // filled in at full resolution in the background while the user is idle.
     int bgHighResSeg;       // next segment index to compute at high-res
     bool bgFinished;        // true when all segments are computed at high-res
-    int lastInteractedFrame; // frame counter when last user interaction occurred
     bool isBgProcessing;    // true while background task is actively computing
 
     // FFT size cache — LRU cache of previously computed STFT results.
@@ -202,6 +208,10 @@ extern Sound AudioPlaybackSound;
 extern Texture2D colormapTexture;
 extern Font mainFont;
 
+// Reset all per-signal state after a new signal is loaded into app.signal
+// (defined in spectrogram.c; used by every load path).
+void ResetForNewSignal(void);
+
 // ============================================================================
 // Small math helpers (header-inline so every module can use them)
 // ============================================================================