refactor: Move platform files to src/platform/ subdirectory

Reorganized platform windowing code into dedicated subdirectory for
better organization and consistency with other subsystems (audio/, gpu/, 3d/).

Changes:
- Created src/platform/ directory
- Moved src/platform.{h,cc} → src/platform/platform.{h,cc}
- Updated 11 include paths: "platform.h" → "platform/platform.h"
  - src/main.cc, src/test_demo.cc
  - src/gpu/gpu.{h,cc}
  - src/platform/platform.cc (self-include)
  - 6 test files
- Updated CMakeLists.txt PLATFORM_SOURCES variable

Verification:
✓ All targets build successfully (demo64k, test_demo, test_platform)
✓ test_platform passes (70% coverage maintained)
✓ demo64k smoke test passed

This completes the platform code reorganization side quest.
No functional changes, purely organizational.

1 files changed, 100 insertions, 0 deletions

diff --git a/src/platform/platform.h b/src/platform/platform.h
new file mode 100644
index 0000000..0a98850
--- /dev/null
+++ b/src/platform/platform.h
@@ -0,0 +1,100 @@
+// Handles windowing, input, and native surface creation.
+// Consolidates platform-specific shims for WebGPU.
+
+#pragma once
+
+#include <cstdint>
+#include <cstring>
+
+// WebGPU specific headers and shims
+#if defined(DEMO_CROSS_COMPILE_WIN32)
+#include <webgpu/webgpu.h>
+#include <webgpu/wgpu.h>
+
+#define WGPUOptionalBool_True true
+#define WGPUOptionalBool_False false
+#define WGPUSType_ShaderSourceWGSL WGPUSType_ShaderModuleWGSLDescriptor
+#define WGPU_DEPTH_SLICE_UNDEFINED 0xffffffff
+#define WGPUSurfaceGetCurrentTextureStatus_SuccessOptimal                      \
+  WGPUSurfaceGetCurrentTextureStatus_Success
+#define WGPUSurfaceGetCurrentTextureStatus_SuccessSuboptimal                   \
+  WGPUSurfaceGetCurrentTextureStatus_Success
+#define WGPUCallbackMode_WaitAnyOnly 0
+
+typedef WGPUShaderModuleWGSLDescriptor WGPUShaderSourceWGSL;
+
+static inline const char* str_view(const char* str) {
+  return str;
+}
+static inline const char* label_view(const char* str) {
+  return str;
+}
+
+static inline void platform_wgpu_wait_any(WGPUInstance instance) {
+  wgpuInstanceProcessEvents(instance);
+}
+static inline void
+platform_wgpu_set_error_callback(WGPUDevice device,
+                                 WGPUErrorCallback callback) {
+  wgpuDeviceSetUncapturedErrorCallback(device, callback, nullptr);
+}
+
+#else
+#include <webgpu.h>
+#include <wgpu.h>
+
+static inline WGPUStringView str_view(const char* str) {
+  if (!str)
+    return {nullptr, 0};
+  return {str, strlen(str)};
+}
+
+static inline WGPUStringView label_view(const char* str) {
+#if !defined(STRIP_ALL)
+  if (!str)
+    return {nullptr, 0};
+  return {str, strlen(str)};
+#else
+  (void)str;
+  return {nullptr, 0};
+#endif
+}
+
+static inline void platform_wgpu_wait_any(WGPUInstance instance) {
+  wgpuInstanceWaitAny(instance, 0, nullptr, 0);
+}
+static inline void
+platform_wgpu_set_error_callback(WGPUDevice device,
+                                 WGPUUncapturedErrorCallback callback) {
+  // Handled in descriptor for new API, but provided for compatibility if needed
+  // elsewhere
+}
+#endif
+
+// Forward declare GLFWwindow to avoid including the full header here.
+struct GLFWwindow;
+
+struct PlatformState {
+  GLFWwindow* window = nullptr;
+  int width = 1280;
+  int height = 720;
+  float aspect_ratio = 1.0f;
+  double time = 0.0;
+  bool is_fullscreen = false;
+  // Store windowed geometry for fullscreen toggle
+  int windowed_x = 0, windowed_y = 0, windowed_w = 0, windowed_h = 0;
+};
+
+// Refactored platform API
+PlatformState platform_init(bool fullscreen, int width, int height);
+void platform_shutdown(PlatformState* state);
+void platform_poll(PlatformState* state);
+bool platform_should_close(PlatformState* state);
+void platform_toggle_fullscreen(PlatformState* state);
+
+// WebGPU specific surface creation
+WGPUSurface platform_create_wgpu_surface(WGPUInstance instance,
+                                         PlatformState* state);
+
+// Global time query (if needed without state)
+double platform_get_time();
\ No newline at end of file