Add SDL2+OpenGL ES 2 renderer backend for Emscripten/WebGL

Implements Phase 1 of renderer abstraction plan:
- SDL2Types.h: SFML-compatible type stubs in sf:: namespace
- SDL2Renderer.h/cpp: OpenGL ES 2 rendering implementation
- EmscriptenStubs.cpp: Stubs for missing POSIX functions (wait3, wait4, wcsftime)

Build system changes:
- Add MCRF_SDL2 compile-time backend selection
- Add Emscripten SDL2 link options (-sUSE_SDL=2, -sFULL_ES2=1)
- Fix LONG_BIT mismatch for Emscripten in pyport.h

Code changes for SDL2/headless compatibility:
- Guard ImGui includes with !MCRF_HEADLESS && !MCRF_SDL2
- Defer GL shader initialization until after context creation

Current status: Python runs in browser, rendering WIP (canvas sizing issues)

Build commands:
  emcmake cmake -DMCRF_SDL2=ON -B build-emscripten
  emmake make -C build-emscripten

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

src/McRFPy_API.cpp

@@ -32,7 +32,8 @@
 #include "PyUniformCollection.h"  // Shader uniform collection (#106)
 #include "McRogueFaceVersion.h"
 #include "GameEngine.h"
-#ifndef MCRF_HEADLESS
+// ImGui is only available for SFML builds
+#if !defined(MCRF_HEADLESS) && !defined(MCRF_SDL2)
 #include "ImGuiConsole.h"
 #endif
 #include "BenchmarkLogger.h"
@@ -1627,7 +1628,7 @@ PyObject* McRFPy_API::_setDevConsole(PyObject* self, PyObject* args) {
         return NULL;
     }
 
-#ifndef MCRF_HEADLESS
+#if !defined(MCRF_HEADLESS) && !defined(MCRF_SDL2)
     ImGuiConsole::setEnabled(enabled);
 #endif
     Py_RETURN_NONE;