Extract MessageLine, Player pack ops, and Level list management

Refactor step 6. MessageLine (was MsgLine) owns the msg/addmsg/endmsg
machinery, wired to its screen, pre---More-- redraw, and input via
attach(); RogueGame keeps one-line msg/addmsgf/endmsg shorthands so
the ~400 call sites are unchanged. Player gains nextPackChar and
removeFromPack (the state half of pack.c leave_pack); leavePack keeps
only the LastPick repeat-command tracking. Level gains ObjectAt
(misc.c find_obj) and AddObject/RemoveObject/AddMonster/RemoveMonster,
replacing direct attach/detach calls on the level lists. Inventory and
pickup UI flows stay on RogueGame: display and orchestration, not
state surgery. Behavior and RNG order unchanged; suite green.

game/rooms.go

@@ -105,7 +105,7 @@ func (g *RogueGame) digRooms() {
 			gold.Flags = Stackable
 			gold.Group = goldGrp
 			gold.Kind = KindGold
-			attachObj(&g.Level.Objects, gold)
+			g.Level.AddObject(gold)
 		}
 		// Put the monster in
 		prob := 25