Linked access pattern pivots in schema-aware query

A focused patch on top of 0.6.0.

Pivots between linked access patterns

The schema-aware query introduced in 0.6.0 had a bug where pivoting from one access pattern to a linked one could fail in some shapes — the linked pattern would either error or carry the wrong key forward. Fixed in this release. Pivots now resolve cleanly across the access-pattern graph the schema describes.