Stockfish 19 (lite, single-threaded) compiled to WebAssembly by Stockfish.js (Nathan Rugg).
It runs in the player's browser for analysis and game review; the Şaxmat server only serves these files.

Licence: GNU General Public License v3 (see COPYING.txt).
Source code: https://github.com/nmrugg/stockfish.js  (release v19.0.1)
             https://github.com/official-stockfish/Stockfish
The files here are unmodified copies of stockfish-19-lite-single.js / .wasm from the npm package stockfish@19.0.1.