feat: Implement Task #76 external library size measurement

- Use ma_backend_null for audio (100-200KB savings)
- Stub platform/gpu abstractions instead of external APIs
- Add DEMO_STRIP_EXTERNAL_LIBS build mode
- Create stub_types.h with minimal WebGPU opaque types
- Add scripts/measure_size.sh for automated measurement

Results: Demo=4.4MB, External=2.0MB (69% vs 31%)

handoff(Claude): Task #76 complete. Binary compiles but doesn't run (size measurement only).

1 files changed, 53 insertions, 0 deletions

diff --git a/src/platform/stub_platform.cc b/src/platform/stub_platform.cc
new file mode 100644
index 0000000..61473a0
--- /dev/null
+++ b/src/platform/stub_platform.cc
@@ -0,0 +1,53 @@
+// Stub platform implementation for size measurement builds.
+// All functions are no-ops. Binary compiles but does NOT run.
+// This file is only compiled when STRIP_EXTERNAL_LIBS is defined.
+
+#if defined(STRIP_EXTERNAL_LIBS)
+
+#include "platform.h"
+#include "stub_types.h"
+
+// Forward declare GLFWwindow stub
+struct GLFWwindow {};
+
+PlatformState platform_init(bool fullscreen, int width, int height) {
+  (void)fullscreen;
+  PlatformState state = {};
+  state.width = width;
+  state.height = height;
+  state.aspect_ratio = (float)width / (float)height;
+  state.window = nullptr;
+  state.time = 0.0;
+  state.is_fullscreen = false;
+  return state;
+}
+
+void platform_shutdown(PlatformState* state) {
+  (void)state;
+}
+
+void platform_poll(PlatformState* state) {
+  (void)state;
+}
+
+bool platform_should_close(PlatformState* state) {
+  (void)state;
+  return false;
+}
+
+void platform_toggle_fullscreen(PlatformState* state) {
+  (void)state;
+}
+
+WGPUSurface platform_create_wgpu_surface(WGPUInstance instance,
+                                         PlatformState* state) {
+  (void)instance;
+  (void)state;
+  return nullptr;
+}
+
+double platform_get_time() {
+  return 0.0;
+}
+
+#endif // STRIP_EXTERNAL_LIBS