Task Order 1: PR05 - Layer Fixture Import Framework

[rhodges]

PR05 Changelog

Summary

This PR introduces the initial fixture import engine for layer-focused imports using UUID-first identity semantics, with strict validation behavior and root-agnostic graph processing foundations for upcoming PR06/PR07/PR09 work.

What Changed

• Added importer implementation in layers/fixture_import.py.
• Implemented UUID-first upsert for layers.layer rows:
  • Update existing records when UUID matches.
  • Create new records when UUID differs, even if source PK collides.
• Implemented strict duplicate UUID conflict detection:
  • Conflicting rows with same (model, uuid) raise ValueError.
• Implemented strict missing-reference validation:
  • Missing or unresolved UUID refs raise ValueError under current strict policy.
• Implemented two-pass relation handling:
  • Pass 1 imports/upserts layer records.
  • Pass 2 resolves and imports layers.multilayerassociation relationships by UUID.
• Applied unscoped manager lookups (all_objects / base manager) for identity resolution to avoid site-scoping false misses.
• Ensured site association behavior under associate_all_sites=True for both new and updated imported layers.
• Kept and wired helper entrypoints for phased rollout:
  • import_layer_rows
  • import_multilayer_rows
  • import_theme_rows

Test Coverage

• UUID-match update with differing source PK.
• Source PK collision with differing UUID creates a new row.
• Relation resolution by UUID (not source PK) for multilayer association linkage.
• New imported layers associated to all sites by default.
• Duplicate UUID conflict handling raises error.
• Missing relation UUID handling raises error under strict policy.
• Tests live in layers/tests/test_fixture_import.py, and the PR05 test set is passing.

Notes

This PR intentionally focuses on PR05 scope (layer import core + strict graph validation primitives). Broader multilayer dimension/value and theme import semantics are left for the planned follow-on PRs.