* term: keep oversized OSC terminators from raising a bell

An OSC longer than the 8 KiB carry buffer (OSC 52 clipboard writes,
inline images) was dropped mid-sequence, so its BEL terminator was then
scanned as a bare bell and marked the pane Unread. Skip the rest of a
dropped OSC through its terminator instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* cli: drop attach output the snapshot already covers

The server registers a viewer before taking the attach snapshot, so a
chunk streamed meanwhile arrives after the response although the
snapshot already contains it. The GUI trims by `output_offset`; the CLI
wrote it twice. Trim the covered prefix the same way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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Co-authored-by: Simone <185146821+Lucenx9@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>