Add tests for catalog reference extraction and SHOW commands

Tests verify that cross-references (call, create, retrieve) are correctly
registered in the refs table, and that SHOW CALLERS/CALLEES/REFERENCES/IMPACT
commands return expected results including transitive caller chains.

Discovered: association refs (RefKindAssociate) defined but never populated.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ako