Stabilize tmux input submission

[kumabook]

Summary

• add a short delay between sending literal text and the Enter key to tmux
• keep submission enabled by default while preventing Enter from being grouped with rapid text input

Why

Terminal UI applications can classify rapidly injected text as pasted input. When Enter immediately follows the text, it can be consumed as part of that input instead of confirming the command, leaving the command unsubmitted.

Impact

Text sent from Reattach is consistently confirmed with Enter. The delay is limited to 100 ms and applies only before submission.

Testing

• rustfmt --edition 2021 --check src/tmux/send.rs
• cargo test (7 passed)
• git diff --check