fix(ugc): served player models keep the client's own collision

With ugc_manifest_models the world left made models out of the LXFML it
sends when a property loads, so the client never built them and had no HKX:
the UGC server makes no physics, and the HKX request got a 404, so served
models had no collision.

Now every model's LXFML is sent and the client builds each one (NIF and
HKX). For the models whose mesh the UGC server made, once the client has
loaded and 3 seconds after, the world sends it the served NIF's checksum and
NotifyClientUGCModelReady: the client drops what it cached for the model and
asks again, downloads the served mesh (its own NIF no longer matches) and
keeps its own HKX (still matching). An HKX request is answered with the LXFML
too, followed by the same switch, at most 3 times per client and model.

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

resources/sharedconfig.ini

@@ -100,8 +100,7 @@ ugc_debounce_seconds=120
 # out, so only set 1 when every player's boot.cfg points at the UGC server. See docs/UgcServer.md.
 ugc_manifest=0
 
-# 0 or 1, with ugc_manifest=1: placed player models the UGC server has made are downloaded from it (their mesh) instead
-# of being built by each client from the LXFML the world sends; models not made yet are still sent as LXFML. The UGC
-# server makes no physics (.hkx): a client that never built a model itself has no collision for it. See
-# docs/UgcServer.md.
+# 0 or 1, with ugc_manifest=1: placed player models the UGC server has made are shown with its mesh. Each client still
+# gets every model's LXFML and builds it first (for its collision: the UGC server makes no physics), then is switched to
+# the served mesh a few seconds after it has loaded. See docs/UgcServer.md.
 ugc_manifest_models=0