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/fight.go

@@ -549,7 +549,7 @@ func (g *RogueGame) removeMon(mp Coord, tp *Monster, waskill bool) {
 
 	g.Level.SetMonsterAt(mp.Y, mp.X, nil)
 	g.mvaddch(mp.Y, mp.X, tp.OldCh)
-	detachMon(&g.Level.Monsters, tp)
+	g.Level.RemoveMonster(tp)
 
 	if tp.On(Targeted) {
 		g.Kamikaze = false