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/sticks.go

@@ -85,7 +85,7 @@ func (g *RogueGame) doZap() {
 				}
 			case WandPolymorph:
 				pp := tp.Pack
-				detachMon(&g.Level.Monsters, tp)
+				g.Level.RemoveMonster(tp)
 
 				if g.seeMonst(tp) {
 					g.mvaddch(y, x, g.Level.Char(y, x))