feat: instant apply for all-entities proxies

Register/Unregister(serializerName, fieldName) now dirty fieldName on every
live entity whose GetSchemaClassname() matches, via NetworkStateChanged — so a
uniform proxy applies (or clears) on the next pack instead of waiting for the
field to change on its own or a full update (round restart). The entity walk
runs only at register/unregister (rare setup), off the hot path. Mirrors the
NetworkStateChanged dirty the entity-scoped Register overload already does.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>