Optional Rust identifier core support (BILILIGHT_RUST_CORE=1)

The application can link allocation-free code built with Rust 1.81.0,
including its core library and compiler_builtins 0.1.109. Associated upstream
copyright and license texts are included alongside this notice. These notices
do not change the license of the application or its own identifier module.

Sources of the preserved texts:
  Rust 1.81.0: LICENSE-MIT, LICENSE-APACHE, COPYRIGHT
  compiler_builtins 0.1.109: LICENSE.txt

Core is rebuilt from the matching rust-src with panic=abort; no Rust std,
allocator, network stack or user/account data is part of the identifier API.
