fix(lexer): handle UTF-8 characters correctly in comments

[cakevm]

• Fix lexer not handling UTF-8 characters correctly in comments.
  • Multi-byte UTF-8 characters (e.g., →) in comments caused incorrect span positions for subsequent tokens.
  • The lexer now properly tracks byte positions instead of character indices.

Example:

// a → b
#define macro MAIN() = takes (0) returns (1) {
// a → b
x123
}

caused:

❯ hnc test.huff -r

thread 'main' (48566897) panicked at crates/utils/src/file/span.rs:55:44:
byte index 65 is not a char boundary; it is inside '→' (bytes 63..66) of `// a → b
#define macro MAIN() = takes (0) returns (1) {
// a → b
x123
}`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace