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, 22 insertions, 0 deletions

diff --git a/scripts/test_headless.sh b/scripts/test_headless.sh
new file mode 100755
index 0000000..a18ed09
--- /dev/null
+++ b/scripts/test_headless.sh
@@ -0,0 +1,22 @@
+#!/bin/bash
+# Test headless mode build and execution
+# Workspace: demo (shared across all workspaces)
+
+set -e
+
+echo "=== Headless Mode Test ==="
+echo
+
+echo "1. Configuring..."
+cmake -S . -B build_headless -DDEMO_HEADLESS=ON
+
+echo
+echo "2. Building..."
+cmake --build build_headless -j4
+
+echo
+echo "3. Running 5s test..."
+./build_headless/demo64k --headless --duration 5
+
+echo
+echo "=== Test Complete ==="