invite reflection when an activity closes

[vyagh]

When a child closes an activity, Jo offers to talk about what they just made. An offer, never a gate: one note at a time, it slips away if ignored, and it never blocks the close. It's earned a real payload plus a minute of focused time (the toolkit's spent-times). Accepting opens the entry with the rail revealed.

Top of the stack: imports timeline and the shared style, calls show_object(reveal_reflection=True) from #1120.

• _InviteNote skips NotificationWindow by choice (different shape, drift-dismiss, thumbnail). No keyboard path yet, an a11y gap I'm naming now. last CssProvider.
• MIN_ACTIVE_SECONDS is tunable; FIND_PROPERTIES is pinned by a test; no network on the close path; shutdown dismisses a live note.
• Upstream bug: sugar3 DSObject.destroy() never removes its 'Updated' match, so the object leaks and later edits block on it.
  Worked around by clearing object_id first; real fix belongs in sugar-toolkit-gtk3,happy to send it.
• Accepted corners: a close right after a crash can fire one spurious note, an in-flight save at Stop can only under-count, and no file written means no note.

256 journal tests, 18 new.
VM Test: 28s resume-and-quit → no note, ~75s of drawing → note with thumbnail and buttons.

New file in Makefile.am and POTFILES.in; main.py starts the trigger.
Series notes in #1111.