Add headless backend for testing and automation

[jserv]

This commit implements a headless graphics backend that renders to shared memory instead of physical display, enabling automated testing, CI/CD integration, and memory analysis without GUI dependencies.

• POSIX shared memory architecture for IPC
• Double buffering (front/back buffers)
• Real-time FPS tracking and statistics
• PNG screenshot capability with built-in encoder
• Event queue with overflow detection (max 64 events)
• Graceful shutdown command

Use cases:

• Automated GUI testing in CI/CD pipelines
• Memory leak detection with Valgrind/ASan
• Performance benchmarking without display overhead
• Screenshot generation for documentation
• Regression testing with pixel-perfect comparison

---

Summary by cubic

Add a headless graphics backend that renders to POSIX shared memory, plus a control tool for screenshots, event injection, status, and graceful shutdown. This enables automated testing, CI checks, and performance/memory analysis without a GUI.

• New Features

  • Shared-memory renderer with double buffering and damage-driven flips; tracks FPS and frame/ event stats.
  • headless-ctl: status, shot, mouse move/down/up, shutdown, monitor.
  • GitHub Actions workflow: builds headless demo, verifies PNG output, checks deterministic rendering, and runs Valgrind leak detection.
  • Build/Kconfig: BACKEND_HEADLESS and TOOL_HEADLESS_CTL; Makefile targets; docs/backends.md.
  • Safety: 64-event queue with overflow detection and coordinate validation; minor type-safety fixes in draw-common and font-edit.

• Migration

  • Enable: make defconfig; use setconfig.py to set BACKEND_HEADLESS=y and TOOL_HEADLESS_CTL=y; make.
  • Run: start demo-headless, then use headless-ctl for status, screenshots, input, and shutdown.

[cubic-dev-ai]

No issues found across 9 files