windows field-path: set capture register (R12) + gamedata site sig

Fills the Windows midhook item RE-grounded. Since GetBitRange is inlined on Windows,
the field-path midhook hooks the WriteFieldList per-field site (gamedata
CFlattenedSerializer::WriteFieldList_FieldPathSite) — right before each field's
BitCopyPrimitive call, where R12 = the current field index (non-volatile, survives the
intervening call; 1:1 with the per-field BitCopy by construction). Windows FieldPathReg
-> R12 (read as an int index, not a CFieldPath pointer — Windows is index-based);
resolve index->name via the serializer leaf map (reverse of ForceResend.WalkLeaves).
Two hardware-verify points documented (R12 index space == leaf DFS numbering; delta-path
site for full coverage) — same class as Linux's leaf-index, not new assumptions.

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