fix(GUI): centered cursor pivots + ibeam over textfields

Card 0e588a (analyst mechanism-correction applied).

Correction honored: the visible pivot is the *image* drawn at mouse.x,mouse.y in drawCursor — all six cursors were corner-pivoted, not just the four missing newCursor args (those only matter on the hardware path).

• FILE_DrawOffsetTable: per-cursor {x,y} offsets applied in drawCursor (8,8 center for arrows/pickaxe; 10,1 for Hand, matching its hardware hotspot)
• Matching newCursor hotspot args added to the four cursors that lacked them (hardware path parity)
• ibeam: getSystemCursor('ibeam') shown while hovering a textfield — TextField.Hovering() exposes a per-frame flag set in Draw and reset in BeginDrawFrame
• Deferred: grabbing-during-drag needs a cross-module window/slider drag registry that doesn't exist — follow-up card material