Modular Code Compatible Posters (#17703)

* poster modularity

* access control, documentation

* properly exclude lewd

* unittest too

* fix

* poster test fix

code/unit_tests/poster_tests.dm

@@ -0,0 +1,24 @@
+/datum/unit_test/posters_shall_have_legal_states
+	name = "POSTERS: All poster decls shall have valid icon and icon overrides"
+
+/datum/unit_test/posters_shall_have_legal_states/start_test()
+	var/failed = 0
+	var/list/all_posters = decls_repository.get_decls_of_type(/decl/poster)
+	all_posters -= decls_repository.get_decl(/decl/poster/lewd) // Dumb exclusion for now. This really needs to become a valid poster instead of an illegaly made base type
+
+	for(var/path in all_posters)
+		var/decl/poster/D = all_posters[path]
+		var/obj/structure/sign/poster/P = /obj/structure/sign/poster // The base poster shows ALL subtypes except /lewd, so all posters should function here regardless!
+		var/icon/I = initial(P.icon)
+		if(D.icon_override)
+			I = D.icon_override
+		if(!(D.icon_state in cached_icon_states(I)))
+			failed += 1
+			log_unit_test("[D.type]: Poster - missing icon_state \"[D.icon_state]\" in \"[I]\", as [D.icon_override ? "override" : "base"] dmi.")
+
+	if(failed)
+		fail("[failed] posters had missing icon_states or bad icon overrides.")
+	else
+		pass("All [all_posters.len] posters have their icon_states and overrides set correctly.")
+
+	return TRUE