Rust frontends for codehelion.
Implements [codehelion_core::frontend::Frontend] for Rust: an
error-tolerant lexer paired with brace-matching unit-boundary detection.
The [ir] module adds the Structural-mode frontend, which parses the file
with a real Rust parser and maps the tree onto the Syntax IR. Nothing here
executes or expands the source; macros and generics pass through as tokens.