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/game.go

@@ -110,7 +110,7 @@ type RogueGame struct {
 
 	// screen / messages
 	scr         *Screen
-	Msgs        MsgLine
+	Msgs        MessageLine
 	statusCache statusCache
 	invPage     invPage // things.c discovery-list pagination statics
 
@@ -169,6 +169,7 @@ func NewGame(cfg Config) *RogueGame {
 	g.InvDescribe = true
 	g.Msgs.SaveMsg = true
 	g.scr = NewScreen(cfg.Term)
+	g.Msgs.attach(g.scr, g.look, g.readchar)
 	g.FileName = cfg.Home + "/rogue.save"
 
 	g.rogueOpts = cfg.RogueOpts