fix: resolve field by decoding CFieldPath token + walking nested serializer

The captured 'field index' is a packed CFieldPath: ((idx+1)<<22)|((idx+1)<<11)|
(idx+1), up to 3-level descent. Old code used it as a flat array index -> name=''.
Now DecodeToken unpacks i0/i1/i2; ResolveFieldName walks serializer+0x10[i0],
descending via child serializer at record+0x08 (confirmed from EncodeField
recursion) for i1/i2, leaf name at rec+0x08. VCOPY logs decoded path=[i0,i1,i2].

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