Address PR #92 review comments

- Fix misleading comment in docs/api/generator.md: cardinality Card(0, 1000)
  is for symmetric roles with multiple friends, not "exactly 2 players"
- Add issubclass() example in docs/api/crud.md to answer question about
  using issubclass() vs isinstance() for polymorphic type checking
- Remove TEST_IMPROVEMENTS_PLAN.md as the plan has been completed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: CaliLuke