PHASE 13.7.B: CLOSED — parent-child indexing complete

Test results (2026-06-12):
  2165 passed, 2 failed (pre-existing), 32 skipped
  No regressions from Phase 13.7.B changes

Delivered:
  - index_helpers.py: ParentChildRegistry, expand_parent_columns()
  - dsl_compiler.py: register_parent_child() 3-parameter API
  - 38 unit tests + 10 integration tests (2.1M real residuals)
  - explore_residuals_v4.py: 8/8 invariance tests, Snapshot working
  - ensure_index_helpers_loaded() with proper error diagnostics
  - C++ libs: libIndexHelpers.so, libO2ResidualHelpers.so

Pre-existing failures (not 13.7.B):
  - test_arrow_bridge::test_dictionary_type_raises
  - test_draw_integration::test_rvec_batch_draw

Author: miranov25