Client: broom mount sync for remote players

[Kheartz]

Remote players (and server NPCs) are now shown riding broomsticks. When the owning client mounts a broom, remotes spawn the matching broom under the proxy, seat the rider on it, and fly it.

Stacks directly on the locomotion sync; no new wire bump, since the Mounted flag and mountId field were already reserved in human_sync.hpp.

Wire / publish (owning client)

• Publishes mount state each tick: the Mounted state flag + which broom as a 1-based allowlist id (mount_records.hpp), read from the pawn's attach-parent class. Allowlist id (not a raw class string) so a proxy can only ever spawn a known broom.
• Writes world velocity only while mounted (on foot it's zeroed so the delta serializer stays quiet).

Remote proxy (ClientHuman)

• On the Mounted rising edge, spawns the allowlisted broom under the the game's playerAttachSocket; on dismount, destroys it and resets the gait latches so on-foot locomotion re-asserts.
• While mounted the proxy is placed by dead-reckoning (position + velocity-age, age clamped to 250 ms) rather than the on-foot snapshot buffer; zero-latency for high-speed flight, with no fly-off on a stalled sender. The attached rider follows the driven broom.
• Seat/facing corrections for BP_RemoteAvatarCCC (lift onto the broom + yaw alignment) as tunable constants.

Test: Human.setMounted(bool, mountId) + setVelocity(x,y,z) script bindings and a /broomnpcs command that mounts spawned NPCs and orbits them airborne, so broom sync can be eyeballed with a single client.