saveCursor

Saves the core-owned cursor state (DECSC, ESC 7).

The saved state includes:

• cursor column

• cursor row

• pen attributes

• pending-wrap state

• origin mode (DECOM)

One save slot exists per screen. Subsequent calls overwrite that slot and restoreCursor restores it later.

This core does not model parser-owned charset designation or shift state (G0..G3, SO, SI), so DECSC does not capture charset state here.