fix(per-client emit): write fake fresh (skip original copy + advance src), not copy-then-rewrite

SUBST-FAKE fired but the wire value didn't change: the emit called the original bit-copy
(writes the real value) then rewound + re-emitted, which didn't take. Per the WriteFieldList
RE, write the fake FRESH: advance the src bf_read cursor (+0x10) past the real value so the
next field reads correctly, and let the field encoder write the fake at dst's current cursor.
Demo per-client int = 1+(client&0x3F) (small, obvious, same byte-length).