Restore NetworkCollision2D class implementation - #1074

- Fixed prefab collection generator excluded folders not matching when saved on another platform or with a trailing slash (#1080).
- Fixed SetLowestMTUs using the last channel's MTU instead of the lowest (#1101).
- Fixed split message pieces being appended to non-empty buffers (#1079).
- Fixed late-joining clients using the most recent global load's replace option instead of the last replacing load (#1035).
- Fixed client prediction TimeManager.OnUpdate subscriptions stacking on pooled objects (#1075).
- Fixed WriteInt64Unpacked writing a packed value instead of 8 bytes (#1072).
- Fixed runtime parent not being set on clients for instantiated spawns (#1058).
- Fixed instantiated spawns parented to a non-first NetworkBehaviour being placed under the wrong transform (#1058).
- Fixed nested scene objects reparenting directly under their parent NetworkObject on clients (#1048).
- Fixed runtime unparenting of scene objects not reaching clients (#1048).
- Fixed obsolete Unity find and scripting-define calls behind version defines; compiles clean on Unity 6.6.
- Fixed jagged arrays being rejected as multidimensional in codegen (#1105).
- Fixed nullable arrays generating an open Nullable type in codegen (#1105).
- Fixed clientHost scene object lookup for scenes loaded more than once (#1081).
- Fixed despawned or pooled NetworkObjects not being written as a null reference (#1078).
- Fixed NetworkCollision2D being commented out (#1074).
- Fixed TimeManager rewriting the project's physics simulation mode and fixed delta time outside play mode (#1077).
- Fixed network HUD canvas buttons being hidden when using the new input system (#1082).
- Fixed OnDespawnServer not running for a connection removed by an observer rebuild while disconnecting (#1107).
- Added level of detail to NetworkTransform.
- Added TimeManager.NormalizedDeltaTime so OnUpdate movement covers the same distance every tick.
- Added NetworkNavMeshAgent, synchronizing a NavMeshAgent.