Import slider relations from layer fixture

[rhodges]

PR07 Summary

Overview

This PR adds TDD coverage and importer execution support for slider-related fixture rows so multilayer graph relationships are preserved on import.

Scope Delivered

• Added PR07 tests for multilayer import integrity in layers/tests/test_fixture_import.py.
• Implemented import handling for:
  • layers.multilayerdimension
  • layers.multilayerdimensionvalue
  • Existing layers.multilayerassociation linkage behavior validated alongside the new rows.
• Enforced strict UUID-based relation resolution across the slider graph:
  • MultilayerDimension.layer
  • MultilayerAssociation.parentLayer
  • MultilayerAssociation.layer
  • MultilayerDimensionValue.dimension
  • MultilayerDimensionValue.associations (M2M)
• Added strict failure behavior for missing multilayer refs under error policy.

Relationship Integrity Guarantees (PR07)

• Dimension rows resolve to the correct parent layer via UUID.
• Association rows resolve both parent layer and target layer via UUID.
• Dimension value rows resolve to the correct dimension via UUID.
• Dimension value M2M association memberships are reconstructed from UUID refs.
• Missing dimension/association refs fail fast with ValueError in strict mode.

Test Outcomes

• Total fixture import tests now passing: 16/16 (as reported).
• PR07-specific tests validate both:
  • Happy path multilayer graph reconstruction.
  • Strict-mode failure path for missing slider refs.

Files Involved

• layers/tests/test_fixture_import.py
• layers/fixture_import.py