[ALL] Predict func_conveyor player movement

[speedvoltage]

Issue:

While looking for some additional prediction errors, I found that the server applies func_conveyor velocity to grounded players in CheckMovingGround, while the equivalent client prediction path is disabled. The client therefore predicts movement without the conveyor's base velocity, causing movement prediction errors and server corrections.

Fix:

Network the conveyor movement state and apply its velocity during client prediction. Rebuild the conveyor state after restore and speed changes so the client and server simulate the same movement.

Before (affects accuracy of my movement here):

2026-08-31.21-38-58.mp4

After (clearing this level much faster and smoothly now):

2026-08-31.21-52-40.mp4