perf: gate per-client hooks on registration emptiness

The per-client substitution detours fire on the hottest paths — ValueCopyHook
runs for every field copy of every entity for every client, and it was walking
the CFieldPath to resolve a field name on each. With no per-client registrations
there's nothing to substitute, so short-circuit at the top of all four capture/
copy hooks (ValueCopyHook, GetBitRangeHook, WflShim, WdeEntityCaptureHook) on
_registry.IsEmpty — the detour then costs one check plus the trampoline. The
uniform encoder hook already gates on _byName.IsEmpty. Correct by construction:
no registrations means the unmodified value is what would have been written.

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