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/scrolls.go

@@ -104,7 +104,7 @@ func (g *RogueGame) readScroll() {
 				// Or anything else nasty
 				if ch := g.Level.VisibleChar(y, x); stepOk(ch) {
 					if ch == Scroll {
-						if fo := g.findObj(y, x); fo != nil && fo.ScrollKind() == ScrollScareMonster {
+						if fo := g.Level.ObjectAt(y, x); fo != nil && fo.ScrollKind() == ScrollScareMonster {
 							continue
 						}
 					}