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

src/platform_web.c

@@ -0,0 +1,65 @@
+/*
+ * platform_web.c — Emscripten/WebAssembly stub implementation
+ *
+ * The web version cannot spawn child processes, so these functions
+ * return "not supported". A real implementation would use Emscripten's
+ * pthreads or asyncify if ffmpeg integration is needed.
+ */
+
+#include "platform.h"
+#include <string.h>
+#include <stdarg.h>
+#include <stdio.h>
+
+/* ── Helpers ─────────────────────────────────────────────────────────── */
+
+static char last_error_msg[256] = { 0 };
+
+static void set_last_error(const char *fmt, ...) {
+    va_list args;
+    va_start(args, fmt);
+    vsnprintf(last_error_msg, sizeof(last_error_msg), fmt, args);
+    va_end(args);
+}
+
+/* ── Platform API ────────────────────────────────────────────────────── */
+
+PlatformError Platform_SpawnChild(const char *program, const char **argv,
+                                  PlatformSpawnHandle *out_handle) {
+    (void)program;
+    (void)argv;
+    set_last_error("Spawning child processes is not supported on the web");
+    out_handle->last_error = ENOSYS;
+    return PLATFORM_NOT_SUPPORTED;
+}
+
+PlatformError Platform_WaitForChild(PlatformSpawnHandle *handle) {
+    (void)handle;
+    return PLATFORM_NOT_SUPPORTED;
+}
+
+SpawnStatus Platform_GetExitStatus(const PlatformSpawnHandle *handle, int *code) {
+    (void)handle;
+    (void)code;
+    return SPAWN_WAITING;
+}
+
+void Platform_CloseSpawnHandle(PlatformSpawnHandle *handle) {
+    if (!handle) return;
+    handle->handle = NULL;
+    handle->status = SPAWN_WAITING;
+    handle->exit_code = 0;
+    handle->last_error = 0;
+}
+
+const char *Platform_GetLastErrorMessage(void) {
+    return last_error_msg;
+}
+
+const char *Platform_GetTempDir(void) {
+    /*
+     * Emscripten's virtual filesystem: "/" works as root.
+     * For persistent downloads/uploads, consider IDBFS mount.
+     */
+    return "/";
+}