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/creature.go

@@ -50,6 +50,47 @@ func (p *Player) IsWearing(ring RingKind) bool {
 	return p.IsRing(Left, ring) || p.IsRing(Right, ring)
 }
 
+// nextPackChar claims and returns the next unused pack character (pack.c
+// pack_char).
+func (p *Player) nextPackChar() byte {
+	for i := range p.PackUsed {
+		if !p.PackUsed[i] {
+			p.PackUsed[i] = true
+
+			return byte(i) + 'a'
+		}
+	}
+
+	return byte(len(p.PackUsed)) + 'a' // C would walk off the array here
+}
+
+// removeFromPack takes an item out of the pack: the whole entry, or one
+// of a stack when all is false (the bookkeeping half of pack.c
+// leave_pack). It returns the object that left the pack — a copy when
+// newobj asks for a split.
+func (p *Player) removeFromPack(obj *Object, newobj, all bool) *Object {
+	p.Inpack--
+
+	nobj := obj
+	if obj.Count > 1 && !all {
+		obj.Count--
+		if obj.Group != 0 {
+			p.Inpack++
+		}
+
+		if newobj {
+			copied := *obj
+			nobj = &copied
+			nobj.Count = 1
+		}
+	} else {
+		p.PackUsed[obj.PackCh-'a'] = false
+		detachObj(&p.Pack, obj)
+	}
+
+	return nobj
+}
+
 // attachMon pushes a monster onto the front of a list (list.c attach).
 func attachMon(list *[]*Monster, item *Monster) {
 	*list = append([]*Monster{item}, *list...)