Plan pure BSL profile management

[hachej]

Summary

• Add implementation plan for making profile management pure BSL without requiring xorq
• Document import-boundary, packaging, test, and docs changes
• Incorporated thermo-nuclear review feedback into the final plan

Validation

• Thermo-nuclear review loop completed; final review found no severe structural blockers
• Attempted: uv run pytest src/boring_semantic_layer/tests/test_profile.py -q
  • Blocked before tests by pyarrow build requiring missing cmake under Python 3.14