Add MCRF_HEADLESS compile-time build option for #158

This commit enables McRogueFace to compile without SFML dependencies
when built with -DMCRF_HEADLESS, a prerequisite for Emscripten/WebAssembly
support.

Changes:
- Add src/platform/HeadlessTypes.h (~900 lines of SFML type stubs)
- Consolidate all SFML includes through src/Common.h (15 files fixed)
- Wrap ImGui-SFML with #ifndef MCRF_HEADLESS guards
- Disable debug console/explorer in headless builds
- Add comprehensive research document: docs/EMSCRIPTEN_RESEARCH.md

The headless build compiles successfully but uses stub implementations
that return failure/no-op. This proves the abstraction boundary is clean
and enables future work on alternative backends (VRSFML, Emscripten).

What still works in headless mode:
- Python interpreter and script execution
- libtcod integrations (pathfinding, FOV, noise, BSP, heightmaps)
- Timer system and scene management
- All game logic and data structures

Build commands:
  Normal:   make
  Headless: cmake .. -DCMAKE_CXX_FLAGS="-DMCRF_HEADLESS" && make

Addresses #158

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/main.cpp

@@ -1,4 +1,4 @@
-#include <SFML/Graphics.hpp>
+#include "Common.h"
 #include "GameEngine.h"
 #include "CommandLineParser.h"
 #include "McRogueFaceConfig.h"