docs+code: Windows field resolution is index-based (item 3 RE conclusion)

RE of Windows WriteFieldList settled the open Windows-midhook-field-path question: the
per-field identity is resolved by binary-searching the field key over the serializer's
sorted field array (index-based), NOT a CFieldPath pointer. So the Linux FieldPathReg
midhook model does not port; Windows per-client substitution needs index->name resolution
(the reverse of ForceResend's flattened-leaf map, which now exists). FieldPathReg pinned
Linux-only (rdi); the Windows midhook can't install anyway (GetBitRange inlined). Updated
docs/FORCE_RESEND.md + the FieldSubstitution comments accordingly.

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