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/Common.h

@@ -1,6 +1,24 @@
 # pragma once
-#include <SFML/Graphics.hpp>
-#include <SFML/Audio.hpp>
+
+// =============================================================================
+// Platform Selection
+// =============================================================================
+// Define MCRF_HEADLESS to build without SFML graphics/audio dependencies.
+// This enables headless operation for servers, CI, and Emscripten builds.
+//
+// Build with: cmake -DMCRF_HEADLESS=ON ..
+// =============================================================================
+
+#ifdef MCRF_HEADLESS
+    // Use headless type stubs instead of SFML
+    #include "platform/HeadlessTypes.h"
+    #define MCRF_GRAPHICS_BACKEND "headless"
+#else
+    // Use SFML for graphics and audio
+    #include <SFML/Graphics.hpp>
+    #include <SFML/Audio.hpp>
+    #define MCRF_GRAPHICS_BACKEND "sfml"
+#endif
 
 // Maximum dimension for grids, layers, and heightmaps (8192x8192 = 256MB of float data)
 // Prevents integer overflow in size calculations and limits memory allocation