feat(property): best friends of the owner can build on the property

property_bff_build (off by default, as live): while the owner is in build
mode, their best friends can join it and place, move and pick up models,
build brick by brick and edit behaviors, each from their own inventory.
Only the owner starts build mode; a best friend still in it when the owner
leaves keeps building until they leave it.

The client lets only the property's owner edit, so each player now gets
their own DownloadPropertyData, with their own id as the owner while they
can build, sent again whenever that changes. SetBuildModeConfirmed and
GetModelsOnProperty go to each player instead of everyone. The first
builder makes the property private and pauses the models; the last one
restores them. Saving no longer needs the owner in the world.

A model taken off the property goes to whoever placed it; if they aren't
here it stays placed. Privacy and the property's name stay owner only
(neither was checked before).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

resources/worldconfig.ini

@@ -81,6 +81,11 @@ cdclient_mismatch_message=We detected that your client is out of date. Please up
 # Auto reject properties which contain no models | must be 1 in order to auto reject.
 auto_reject_empty_properties=0
 
+# 1: best friends of a property's owner can join the owner's build mode on the property (place, move and pick up
+# models, brick by brick building, behaviors). Only the owner starts build mode; best friends in it stay until they
+# leave it. 0 (as live): only the owner builds.
+property_bff_build=0
+
 # comma delimited list of items to not drop in hardcore mode
 hardcore_excluded_item_drops=6086,7044