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_web.sh

@@ -42,9 +42,22 @@ cd "$RAYLIB_BUILD"
 emar rcs libraylib.a rcore.o rshapes.o rtextures.o rtext.o rmodels.o raudio.o
 cd "$SCRIPT_DIR"
 
-echo "=== Step 2: Compiling spectrogram for web ==="
+# Compile spectrogram (web version — no subprocess support)
+# platform_web.c provides stub implementations for spawn/error functions
+echo "=== Step 2: Compiling spectrogram + platform for web ==="
 cd "$SCRIPT_DIR"
 
+# Compile platform_web.o (handles fork/execvp stubs for web)
+emcc -c -Os -Wall -DPLATFORM_WEB "$SRC_DIR/platform_web.c" -o "$RAYLIB_BUILD/platform_web.o"
+
+# Compile spectrogram
+emcc -c -Os -Wall -DPLATFORM_WEB "$SRC_DIR/spectrogram.c" \
+    -I"$RAYLIB_DIR/src" \
+    -I"$RAYLIB_DIR/src/external" \
+    -I"$RAYLIB_DIR/src/external/glfw/include" \
+    -Iinclude \
+    -o "$RAYLIB_BUILD/spectrogram.o"
+
 # Linker flags
 LDFLAGS="-s USE_GLFW=3 -s ASYNCIFY -s TOTAL_MEMORY=67108864 -s FORCE_FILESYSTEM=1 --preload-file resources@resources --preload-file fonts/DejaVuSansMono.ttf@fonts/DejaVuSansMono.ttf --shell-file $SCRIPT_DIR/web_shell.html -s NO_EXIT_RUNTIME=1"
 
@@ -56,7 +69,8 @@ fi
 
 # Compile and link
 emcc -o "$OUTPUT" \
-    "$SRC_DIR/spectrogram.c" \
+    "$RAYLIB_BUILD/platform_web.o" \
+    "$RAYLIB_BUILD/spectrogram.o" \
     -I"$RAYLIB_DIR/src" \
     -I"$RAYLIB_DIR/src/external" \
     -I"$RAYLIB_DIR/src/external/glfw/include" \