When AppKit delivers a single C0 control character during
marked-text composition, Ghostty should treat it as input consumed by
the composing state instead of forwarding it to the terminal.
This prevents control-key IME actions, such as Japanese input
shortcuts like ctrl+h/j/m/n, from leaking into the terminal while
composition is still active. Printable text and non-composing control
input continue through the normal key path.
AI usage: OpenAI Codex helped investigate, implement, test, and refine
this change. I reviewed and tested the resulting code.
