[Bug] Panic on invalid UTF-8 byte index with full-width Japanese punctuation

Bug Description
A runtime panic occurs when processing Japanese text containing full-width punctuation marks. The error indicates an invalid byte index access within a multi-byte UTF-8 character.
thread '<unnamed>' (1030363) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 14 is not a char boundary; it is inside '）' (bytes 12..15) of 使わない）

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.5
• Feedback ID: 9ad46877-d129-4af7-95e5-e2b08d718ed5

Errors

[{"error":"Error: NON-FATAL: Lock acquisition failed for /Users/tsubasa/.local/share/claude/versions/2.1.5 (expected in multi-process scenarios)\n    at lhR (/$bunfs/root/claude:2770:2157)\n    at yEA (/$bunfs/root/claude:2770:1268)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-01-13T01:21:59.998Z"}]