docs: add Option D implementation anchors (CalcDelta vtable-slot-8 hook target)

RE'd the exact hook target for the recommended force-resend: CalcDelta is a virtual
call on the serializer singleton — vtable slot 8 of *(global 0x00ac4ae0) inside
WriteDeltaEntity_Internal (libengine2 0x7d15c0 / engine2.dll 0x1800d1240). Documents
the IVirtualHook resolution (global-load sig + '+3 r' factory -> object -> vtable[8]),
the field-index-append hook body, and the field-name->index cache from the full-update
path. Ready to build gated + validate on a live client.

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