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/ImGuiSceneExplorer.cpp

@@ -1,3 +1,8 @@
+// ImGuiSceneExplorer.cpp - Debug scene hierarchy explorer using ImGui
+// This file is excluded from headless builds (no GUI/debug interface needed)
+
+#ifndef MCRF_HEADLESS
+
 #include "ImGuiSceneExplorer.h"
 #include "imgui.h"
 #include "GameEngine.h"
@@ -283,3 +288,5 @@ const char* ImGuiSceneExplorer::getTypeName(UIDrawable* drawable) {
         default:                       return "Unknown";
     }
 }
+
+#endif // MCRF_HEADLESS