Add rebound recipe (librebound + rebound)

[MMesch]

Add emscripten-forge recipes for REBOUND as a single multi-output recipe (following the matplotlib pattern):

librebound — C shared library (side module)

• Builds src/*.c as a WASM side module with -sEXPORT_ALL=1
• Installs to $PREFIX/lib/librebound.so and $PREFIX/include/
• C test compiles test + source files statically (side module .so cannot be linked by wasm-ld)

rebound — Python ctypes wrapper (depends on librebound)

• Patches setup.py to skip C extension build on emscripten
• Patches rebound/__init__.py to load librebound.so from $PREFIX/lib/ via ctypes
• Patches rebound/horizons.py to use try/except ImportError around pyodide.http import instead of sys.modules check
• Patches src/simulation.c to remove emscripten_sleep() calls in the integration loop and pause poll — these require -sASYNCIFY which we do not compile with (no display server); without this patch sim.integrate() aborts in a browser context
• Tested with pytester: import + 2-body N-body simulation

[IsabelParedes]

@MMesch Since both recipes have the same source and are related, both packages can be in a single recipe rebound/recipe.yaml with multiple outputs. See matplotlib for example

• https://github.com/emscripten-forge/recipes/blob/main/recipes/recipes_emscripten/matplotlib/recipe.yaml

[MMesch]

Ok great. I'll do that!