fix(engine): module doc to reflect schnellru::LruMap backend

[VolodymyrBg]

• Replaced an outdated reference to “moka” with the correct schnellru::LruMap in the module-level documentation of
  crates/engine/tree/src/tree/precompile_cache.rs
• The implementation imports and uses schnellru::LruMap (with ByLength limiter) and parking_lot::Mutex; other comments in the file already describe LruMap semantics.
• This change ensures the documentation accurately reflects the current implementation. No functional code changes.

[mattsse]

let this one slide, because actually a valid doc fix