Introduce a minimal paste-burst heuristic in the chat composer so Enter
is treated as a newline during paste-like bursts (plain chars arriving
in very short intervals), avoiding premature submit after the first line
on Windows consoles that lack bracketed paste.
- Detect tight sequences of plain Char events; open a short window where
Enter inserts a newline instead of submitting.
- Extend the window on newline to handle blank lines in pasted content.
- No behavior change for terminals that already emit Event::Paste; no
OS/env toggles added.
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