feat(line-editor): support bracketed paste mode

What

Adds bracketed-paste support to the interactive line editor.

Pasting multi-line text without bracketed paste is a classic footgun: each embedded newline submits a line, so a pasted block runs partially, line by line — often executing something the user hadn't reviewed. Den had no paste handling at all.

• Enable the mode (ESC[?2004h on entry / ESC[?2004l on exit) so the terminal wraps pastes in ESC[200~ … ESC[201~.
• types.zig: add paste_start / paste_end to EscapeSequence.
• escape.zig: map CSI 200~ / 201~ (the existing ESC[N~ number parser already does the work — two switch arms).
• line_editor.zig: handlePaste reads everything up to the terminator via a small state machine and inserts it in one batch (insertPasteByte, single redraw), normalizing embedded newlines/tabs to spaces so the whole paste stays on one editable line and only runs on Enter.

Verification

• zig build clean (the exhaustive EscapeSequence switches forced both handlers to cover the new variants).
• PTY test: pasting echo LINE1\necho DANGER2 bracketed by ESC[200~/201~ yields the single line echo LINE1 echo DANGER2; standalone echo LINE1 is never executed; ESC[?2004h/l are emitted.

Three small, additive files; no API changes.