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/misc.go

@@ -223,17 +223,6 @@ func (g *RogueGame) showFloor() bool {
 	return true
 }
 
-// findObj finds the unclaimed object at (y, x) (misc.c find_obj).
-func (g *RogueGame) findObj(y, x int) *Object {
-	for _, obj := range g.Level.Objects {
-		if obj.Pos.Y == y && obj.Pos.X == x {
-			return obj
-		}
-	}
-
-	return nil
-}
-
 // eat lets her try to eat something (misc.c eat).
 func (g *RogueGame) eat() {
 	obj, ok := g.promptPackItem("eat", KindFood)