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/stft.c

@@ -371,6 +371,10 @@ int ComputeSkipFactor(float signalDurationSec)
 // ===== Amplitude auto-scaling =====
 void AutoScaleAmplitude(StftResult* stft)
 {
+    // Don't clobber a dB floor the user has set by hand. Reset on new file load
+    // re-enables auto-scaling (see ResetForNewSignal).
+    if (app.amplitudeUserSet) return;
+
     float maxDb = -999.0f;
     float minDb = 0.0f;
     for (int seg = 0; seg < stft->numSegments; seg++) {