Reptides Arcade engine source

Engine commit: 50d79c41610cf06b72c3fe6bcdf6fb684014d8ff
Playable shell commit: c83556617
Based on The Powder Toy. GPL-3.0; see LICENSE.txt and the source archive.
Complete committed engine source: engine-source-50d79c41.tar.gz
Shell sources: app.js, style.css, index.html and material-icons.js/.css here.
Production integration changes the asset base, lab label, stylesheet cache key
and adds these source/download notices. Simulation code is unchanged.

Build requirements: Emscripten 3.1.72, Meson, Ninja, Python 3.
Activate emsdk_env.sh so em++, emar and emstrip are on PATH.
The checked Meson wraps retrieve pinned tpt-libs dependencies.
Dependency project/source: https://github.com/The-Powder-Toy/tpt-libs

From the extracted engine directory, desktop:
meson setup build-web --cross-file=.github/emscripten-ghactions.ini -Dstatic=prebuilt -Dbuildtype=release -Dhttp=false -Dshared_data_folder=false -Dcan_install=no -Dweb_threads=false -Dcpp_args=[] -Dapp_name='Physics Sim' -Dapp_exe=physics-sim -Dapp_id=co.reptides.physicssim -Dapp_data=PhysicsSim
ninja -C build-web

Portrait uses the same setup with build-web-portrait and
-Dcpp_args="['-DPHYSICS_SIM_PORTRAIT']" instead.
Keep physics-sim.js and physics-sim.wasm together in each build directory.

The packaged binaries are the supplied committed builds; this import does not
recompile or retune the simulation. The archive preserves every file in the
engine commit, including its upstream build instructions and licence.