fix(xserv-chat): UTF-8/CJK-aware line input

Cooked-mode read_line() left line editing to the terminal, so Backspace on a
multi-byte 汉字/かな/한글 deleted a byte (or behaved inconsistently across TTYs).
Replace with a raw-mode reader (libc termios): Backspace pops a whole char,
multi-byte input is reassembled from its continuation bytes, and a full-line
redraw renders double-width glyphs correctly. Non-TTY input falls back to a
plain read; raw mode is restored after each line. libc is already a locked
transitive dep, so this builds offline.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.toml

@@ -18,6 +18,7 @@ license = "MIT"
 [workspace.dependencies]
 half = "2"
 smallvec = "1"
+libc = "0.2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 safetensors = "0.5"