Add three Lance surface guards de-risking a future persisted-adjacency cache:
- a compile-only guard pinning the fragment physical_rows + index-detail
surface that key_column_index_coverage mirrors (the C6 fallback);
- a runtime probe confirming a scalar BTREE on the system column
_row_last_updated_at_version is not buildable via the normal create-index
path (the column is not in the user schema), so a version-column range delta
is not viable as drafted;
- a runtime probe confirming per-fragment deletion metadata
(deletion_file.num_deleted_rows) is available as cheap O(fragments) metadata,
the primitive a fragment-coverage delete model would rely on.
The probes turn the two largest substrate assumptions into green/red CI facts
before any cache work begins.