Refactor: extract platform abstraction for subprocess spawning

Move all Unix-specific syscalls (fork, execvp, waitpid, strerror, /tmp)
into a platform layer so the same spectrogram.c can target Windows
(CreateProcess) and WebAssembly (stubs). Key changes:

- src/platform.h: public API with spawn handle, error codes, path helpers
- src/platform_linux.c: fork/execvp implementation, /tmp, strerror
- src/platform_win32.c: CreateProcess implementation, temp dir lookup
- src/platform_web.c: stub implementations (no subprocess support on web)
- src/spectrogram.c: consume only platform.h; replace pid_t/fork/execvp
  with Platform_SpawnChild/WaitForChild; use Platform_GetTempDir() for
  /tmp path; replace strdup with malloc/memcpy for portability
- Build: add platform_*.c to gmake, Premake, and build_web.sh

build/premake5.lua

@@ -202,6 +202,19 @@ if (downloadRaylib) then
             files {"../src/*.rc", "../src/*.ico"}
             files {"../resources/**"}
 
+        -- Exclude platform-specific files on each target
+        filter {"system:linux"}
+            excludedfiles { "../src/platform_win32.c", "../src/platform_web.c" }
+
+        filter {"system:windows"}
+            excludedfiles { "../src/platform_linux.c", "../src/platform_web.c" }
+
+        filter {"system:macosx"}
+            excludedfiles { "../src/platform_win32.c", "../src/platform_web.c" }
+
+        filter {"action:emscripten"}
+            excludedfiles { "../src/platform_linux.c", "../src/platform_win32.c" }
+
         filter{}
         
         includedirs { "../src" }