fix: plug memory leaks on file reload and texture regen

Three leaks, all at the relevant choke points:

- ComputeSTFTInit reallocated result->segments without freeing the old
  array, and runs twice per load (load site + loadingPhase 0), so every
  file load leaked the STFT. Free the previous result at the top.
- LoadWavFile malloc'd signal->samples without freeing the previous
  buffer; free it just before the alloc (after all failure returns, so a
  failed load never frees the existing signal).
- GenerateSpectrogramTexture overwrote *image with a fresh GenImageColor
  without unloading the old one — leaked an image buffer on every reload
  and every dB-floor/colormap regen. Unload first (NULL-safe on first use).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/spectrogram.c

@@ -614,6 +614,7 @@ static void ApplyHannWindow(float* samples, int n)
 
 static void ComputeSTFTInit(AudioSignal* signal, StftResult* result, int fftSize)
 {
+    FreeSTFT(result);  // release any previous result before reallocating
     int hopSize = fftSize / HOP_RATIO;  // 75% overlap
     int numSegments = (signal->numSamples - fftSize) / hopSize + 1;
     if (numSegments <= 0) numSegments = 1;
@@ -913,6 +914,7 @@ static bool LoadWavFile(const char* filepath, AudioSignal* signal)
     signal->channels = wave.channels;
     signal->numSamples = wave.frameCount * wave.channels;
     signal->duration = (float)wave.frameCount / wave.sampleRate;
+    if (signal->samples) free(signal->samples);  // free previous file's samples
     signal->samples = (float*)malloc(signal->numSamples * sizeof(float));
     
     if (wave.sampleSize == 16) {
@@ -965,6 +967,7 @@ static void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D
     int fftSize = (height - 1) * 2;
     float freqPerBin = (float)stft->sampleRate / fftSize;
 
+    UnloadImage(*image);  // release previous image (NULL-safe on first call)
     *image = GenImageColor(width, height, BLACK);
     Color* pixels = (Color*)image->data;