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/ImGuiSceneExplorer.h

@@ -1,7 +1,7 @@
 #pragma once
 
-// ImGuiSceneExplorer - excluded from headless builds (no GUI/debug interface)
-#ifndef MCRF_HEADLESS
+// ImGuiSceneExplorer - excluded from headless and SDL2 builds (ImGui-SFML only)
+#if !defined(MCRF_HEADLESS) && !defined(MCRF_SDL2)
 
 #include <string>
 #include <memory>