Enable auxiliary columns for rescore, IVF, and DiskANN indexes

The constructor previously rejected auxiliary columns (+col) for all
non-flat index types. Analysis confirms all code paths already handle
aux columns correctly — aux data lives in _auxiliary shadow table,
independent of the vector index structures.

Remove the three auxiliary column guards. Metadata and partition key
guards remain in place (separate analysis needed).

Adds 8 snapshot-based tests covering shadow table creation, insert+KNN
returning aux values, aux UPDATE, aux DELETE cleanup, and DROP TABLE
for both rescore and DiskANN. IVF aux verified with IVF-enabled build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>