feat: Replace libtcod with libtcod-headless fork (closes #134)

Replace upstream libtcod with jmccardle/libtcod-headless fork that:
- Builds without SDL dependency (NO_SDL compile flag)
- Uses vendored dependencies (lodepng, utf8proc, stb)
- Provides all core algorithms (FOV, pathfinding, BSP, noise)

Changes:
- Update .gitmodules to use libtcod-headless (2.2.1-headless branch)
- Add NO_SDL compile definition to CMakeLists.txt
- Remove old libtcod submodule

Build instructions: deps/libtcod symlink should point to
modules/libtcod-headless/src/libtcod (configured during build setup)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CMakeLists.txt

@@ -50,6 +50,9 @@ link_directories(${CMAKE_SOURCE_DIR}/__lib)
 # Define the executable target before linking libraries
 add_executable(mcrogueface ${SOURCES})
 
+# Define NO_SDL for libtcod-headless headers (excludes SDL-dependent code)
+target_compile_definitions(mcrogueface PRIVATE NO_SDL)
+
 # On Windows, set subsystem to WINDOWS to hide console
 if(WIN32)
     set_target_properties(mcrogueface PROPERTIES