UTF-8 byte index boundary error with Korean characters

Description

When processing Korean text, a Rust panic occurs due to UTF-8 byte index boundary issues.

Error Message

byte index 11 is not a char boundary; it is inside '딩' (bytes 9..12) of `트레이딩`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Context

• The Korean word 트레이딩 (meaning "trading") causes the error
• Korean characters are 3 bytes each in UTF-8
• The error occurs when slicing at byte index 11, which falls in the middle of the character '딩' (bytes 9-12)

Expected Behavior

String operations should use character boundaries rather than raw byte indices when handling multi-byte UTF-8 characters.

Environment

• Platform: Windows (win32)
• Claude Code version: Latest

Suggested Fix

Use Rust's char_indices() or similar character-aware methods instead of byte indexing for string slicing operations.