Add ContinuousChannel field to NetworkAnimator for split-channel sync (events stay reliable, continuous values opt-in unreliable)

[isaiahgrant]

Problem

In games with many concurrent NetworkAnimators (zombie hordes, large-scale RTS, etc.), continuous parameter sync (floats, layer weights, speed) generates steady reliable-channel pressure on every tick. With enough instances this saturates the reliable lane on lossy transports — in our case we hit Steam P2P send-buffer overflow during high-density frames, eventually causing connections to drop.

Our take: The reliable channel isn't the right fit for continuous interpolated values: a missed packet doesn't matter, the next tick supersedes it. But triggers and state changes are one-shots that do need to arrive.

Solution

Add a per-instance ContinuousChannel field on NetworkAnimator that lets you opt continuous values onto Unreliable while keeping triggers, state changes, and crossfades on Reliable.

• Continuous values (bool/float/int parameters, layer weights, speed) → _continuousChannel
• Events (triggers, STATE, CROSSFADE) → always Reliable

Default is Channel.Reliable, which routes through the original code path bit-for-bit unchanged. Existing projects see no behavior change. The split-channel path is only entered when an instance opts in.

ContinuousChannel is exposed as a public property and can be flipped at runtime — useful for distance-based importance (Reliable when close to a player, Unreliable when far).

Implementation notes

• New AnimatorUpdatedSplit writes to two pooled writers in parallel — same logic as AnimatorUpdated, just routed by category.
• Two new TargetRpcs and two new ServerRpcs carry the streams. Receive logic is shared with the existing path via
  ReceiveTargetAnimatorPayload / ReceiveServerAnimatorPayload, so deserialization stays identical.
• No changes to wire format on the existing path. No changes to existing public API.

Testing

Tested in a 4-player co-op zombie game with up to 300 concurrent NetworkAnimators per host. Reliable-lane pressure on the host's outbound traffic dropped substantially with ContinuousChannel = Unreliable, and Steam P2P disconnects during burst spawns stopped reproducing. Triggers (hit reactions, attack states) still arrive reliably.