Root cause: _current_line_start was set after \r\n but BEFORE the prompt
was rendered, so Backspace could erase prompt characters after reaching
the input line start.
Fix: send \x7f (DEL) to shell on Backspace and \x1b[3~ on Delete instead
of locally deleting characters. The shell's readline handles backspace
correctly - it ignores DEL at the input line start, so the prompt is
never touched. The existing \b handler in _on_data already correctly
processes the shell's \b-space-\b echo sequence.
Verified: typing 'abc' + 3 Backspace deletes input, 5 more Backspace
leaves prompt 'root@host:~# ' intact.
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