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/newlevel.go

@@ -105,7 +105,7 @@ func (g *RogueGame) putThings() {
 		if g.rnd(100) < 36 {
 			// Pick a new object and link it in the list
 			obj := g.newThing()
-			attachObj(&g.Level.Objects, obj)
+			g.Level.AddObject(obj)
 			// Put it somewhere
 			obj.Pos, _ = g.findFloor(false)
 			g.Level.SetChar(obj.Pos.Y, obj.Pos.X, obj.Kind.Glyph())
@@ -115,7 +115,7 @@ func (g *RogueGame) putThings() {
 	// yet, put it somewhere on the ground
 	if g.Depth >= AmuletLevel && !g.HasAmulet {
 		obj := newObject()
-		attachObj(&g.Level.Objects, obj)
+		g.Level.AddObject(obj)
 		obj.Damage = dice("0x0")
 		obj.HurlDmg = dice("0x0")
 		obj.ArmorClass = 11
@@ -137,7 +137,7 @@ func (g *RogueGame) treasureRoom() {
 		mp, _ := g.findFloorIn(rp, 2*maxTries, false)
 		tp := g.newThing()
 		tp.Pos = mp
-		attachObj(&g.Level.Objects, tp)
+		g.Level.AddObject(tp)
 		g.Level.SetChar(mp.Y, mp.X, tp.Kind.Glyph())
 	}