feat: Add headless mode for testing without GPU

Implements DEMO_HEADLESS build option for fast iteration cycles:
- Functional GPU/platform stubs (not pure no-ops like STRIP_EXTERNAL_LIBS)
- Audio and timeline systems work normally
- No rendering overhead
- Useful for CI, audio development, timeline validation

Files added:
- doc/HEADLESS_MODE.md - Documentation
- src/gpu/headless_gpu.cc - Validated GPU stubs
- src/platform/headless_platform.cc - Time simulation (60Hz)
- scripts/test_headless.sh - End-to-end test script

Usage:
  cmake -B build_headless -DDEMO_HEADLESS=ON
  cmake --build build_headless -j4
  ./build_headless/demo64k --headless --duration 30

Progress printed every 5s. Compatible with --dump_wav mode.

handoff(Claude): Task #76 follow-up - headless mode complete

1 files changed, 60 insertions, 0 deletions

diff --git a/src/platform/headless_platform.cc b/src/platform/headless_platform.cc
new file mode 100644
index 0000000..4ec8c5d
--- /dev/null
+++ b/src/platform/headless_platform.cc
@@ -0,0 +1,60 @@
+// Headless platform - time simulation without window
+// Workspace: demo (shared across all workspaces)
+
+#if defined(DEMO_HEADLESS)
+
+#include "platform.h"
+#include "stub_types.h"
+#include <stdio.h>
+
+static double g_virtual_time = 0.0;
+static bool g_should_close = false;
+static const double FRAME_TIME = 1.0 / 60.0;
+
+PlatformState platform_init(bool fullscreen, int width, int height) {
+  (void)fullscreen;
+  g_virtual_time = 0.0;
+  g_should_close = false;
+  printf("[headless] Platform initialized (simulated %dx%d)\n", width, height);
+
+  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;
+  printf("[headless] Platform shutdown\n");
+}
+
+void platform_poll(PlatformState* state) {
+  (void)state;
+  g_virtual_time += FRAME_TIME;
+}
+
+bool platform_should_close(PlatformState* state) {
+  (void)state;
+  return g_should_close;
+}
+
+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 g_virtual_time;
+}
+
+#endif // DEMO_HEADLESS