feat: Phase-2 step1 RecipientCapture probe (per-client encode entry)

Detour on the per-client encode fn (engine 0x8fae40, sig in gamedata) that gets
CServerSideClient* in rsi. Captures it into a [ThreadStatic] _currentClient set
before the trampoline + cleared after (try/finally) — so the per-client
WriteFieldList writes within that call (same thread) can read the recipient.
sp_recipcap probe logs client ptr + thread id (first 30) to verify per-client +
worker-thread behavior before wiring substitution.

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