refactor: split spectrogram.c into per-concern modules

spectrogram.c was ~2950 lines holding everything. Break it into cohesive
translation units; spectrogram.c keeps only globals + the main frame loop.

New modules:
- spectrogram_types.h  shared types, constants, extern globals, inline math
- fft.c/.h             FFT, bit-reverse, twiddle (standalone, no app deps)
- stft.c/.h            STFT compute, adaptive resolution, FFT-size LRU cache
- audio.c/.h           WAV/ffmpeg load, FreeSignal, bandpass, playback
- render.c/.h          UI scaling, colormaps, texture gen, on-screen drawing
- ui.c/.h              file browser, sidebar, sliders, PNG export

Also:
- utils.c now includes utils.h instead of re-typedef'ing AudioSignal/
  SignalStats (they had to be hand-synced before).
- Remove dead code: ApplyHannWindow and ComputeSTFTHighResRange were never
  called (the live high-res path is ComputeNextHighResChunk).
- Delete the unused raylib-template main.c.
- rspektrum.make: build the new units. premake5.lua: glob src/**.c so a
  future regen stays correct.

Pure code movement otherwise; no behavior change. Builds clean (-Wshadow).

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

src/render.h

@@ -0,0 +1,25 @@
+// render.h - UI scaling, colormaps, spectrogram texture generation, drawing
+#ifndef RENDER_H
+#define RENDER_H
+
+#include "spectrogram_types.h"
+
+// --- UI scaling & scaled text ---
+float GetUIScale(void);
+void  DrawTextScaled(const char* text, float x, float y, float baseSize, Color color);
+float MeasureTextScaled(const char* text, float baseSize);
+
+// --- Colormaps ---
+void GenerateColormapTexture(void);
+
+// --- Spectrogram texture ---
+void GenerateSpectrogramTexture(StftResult* stft, Image* image, Texture2D* texture);
+
+// --- On-screen drawing (operate on the global app state) ---
+void DrawSpectrogramGrid(Rectangle bounds, int numCellsX, int numCellsY, Color color);
+void DrawLabels(Rectangle bounds);
+void DrawSelection(Rectangle bounds);
+void DrawSelectionDrag(Rectangle bounds);
+void DrawPlayhead(Rectangle bounds);
+
+#endif // RENDER_H