Review follow-up on #1658:
- The drag duplicate cleared `ph` without detaching, so a title or body
dragged off with Alt lost its size and bullets. Copy, Duplicate and the
drag now share `detach_on_slide`.
- Detaching at copy time turned a slide placeholder pasted in Slide Master
view into a regular shape. The clipboard keeps the originals, and a paste
there takes them, so it stays a placeholder as before this PR. Keep
Source Formatting bakes them too.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>