Client: remote proxy locomotion (gait + in-air) via snapshot interpolation

[Kheartz]

Remote-player/NPC proxies now animate their on-foot movement (idle/walk/run/sprint blended by ground speed, plus an in-air fall state) and move smoothly instead of snapping between packets.

Locomotion driver (ClientHuman)

• Ground speed derived from the per-packet position delta (EWMA Σdist/Σtime, so packet-timing jitter cancels and the gait clip doesn't flicker).
• Primary path: drive the pak's ABP_RemoteAvatar 1D blendspace (Speed + bInAir) for smooth gait blending.
• Fallback: single-node Hu_BM_* clips / move-blendspace (proxy_locomotion.{h,cpp}) if the pak ABP isn't available, so a build without it still animates.
• Async-build gate so we don't drive a half-built CCC mesh.

Smooth movement — new snapshot interpolation (core/snapshot_interpolator.h)

• The framework's Utils::Interpolator is a per-packet error-chaser that snaps then freezes between packets (and was being fed delay in seconds where it wants milliseconds) → visible jitter.
• Replaced it for humans with a standalone, header-only snapshot/playle): record timestamped (pos, rot) samples,

Wire (shared/modules/human_sync.hpp, HumanEntity)

• New per-tick stateFlags byte + UpdateData on SerializeFields; only InAir is wired now. The Mounted/Cast/Lumos flags and mountId/spellId/aimPitch fields are reserved so broom/spell land additively without re-cutting the wire.

Test includes a /walknpcs to test the gait animations on velocity and visually affirming the interpolation.