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/GameEngine.h

@@ -10,7 +10,8 @@
 #include "HeadlessRenderer.h"
 #include "SceneTransition.h"
 #include "Profiler.h"
-#ifndef MCRF_HEADLESS
+// ImGui is only available for SFML builds (not headless, not SDL2)
+#if !defined(MCRF_HEADLESS) && !defined(MCRF_SDL2)
 #include "ImGuiConsole.h"
 #include "ImGuiSceneExplorer.h"
 #endif
@@ -196,7 +197,8 @@ private:
     int overlayUpdateCounter = 0;             // Only update overlay every N frames
     ProfilerOverlay* profilerOverlay = nullptr; // The actual overlay renderer
 
-#ifndef MCRF_HEADLESS
+// ImGui is only available for SFML builds
+#if !defined(MCRF_HEADLESS) && !defined(MCRF_SDL2)
     // ImGui console overlay
     ImGuiConsole console;
     ImGuiSceneExplorer sceneExplorer;