[area:grid] A second Ctrl+V after a cut pastes the cut text again, from the system clipboard #237

Found by the PO while reviewing #236 (#51), production build of a314bbd, 1280, clipboard permission granted. The same on main before #236: it already cleared the internal clipboard after a cut was pasted.

Repro

1. Type 1 in A1. Ctrl+X.
2. Select D1, Ctrl+V → D1 = 1, A1 emptied (a move, correct).
3. Select A6, Ctrl+V again → A6 = 1.

The internal clipboard is cleared after the cut is pasted, as in Excel, but the text Ctrl+X wrote to the system clipboard is still there, so the second paste falls through to it and goes down as a plain-text copy. In Excel a cut can be pasted once; the second Ctrl+V does nothing.

Nothing is lost or silently wrong (the value appears where the person pasted), hence sev:low.

Done when

• After a cut has been pasted inside the app, a Ctrl+V whose system-clipboard text is that same cut's text pastes nothing (or clear the system clipboard on that paste, if the browser allows it without a prompt — say which in the PR).
• Text copied from another app after the cut still pastes normally.
• A test for each.