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>

build/premake5.lua

@@ -196,7 +196,7 @@ if (downloadRaylib) then
             ["Game Resource Files/*"] = {"../resources/**"},
         }
 
-        files {"../src/spectrogram.c", "../src/**.h", "../src/**.hpp", "../include/**.h", "../include/**.hpp"}
+        files {"../src/**.c", "../src/**.h", "../src/**.hpp", "../include/**.h", "../include/**.hpp"}
         
         filter {"system:windows", "action:vs*"}
             files {"../src/*.rc", "../src/*.ico"}