test(task-31-b2): lock description-free merge compat

[earayu]

scope

Task #31 Phase B2: add cross-backend compatibility coverage for LineageEntityMerger.merge_entities_apply_description_free() in tests/integration/compat/test_lineage_graph_compat.py.

This pins the task #31 § 3.1.4 / § 5.2.b apply contract over the real LineageGraphStore backends (PostgreSQL / Neo4j / Nebula via the existing compat fixture):

• source lineage members re-anchor under the target
• source rows are deleted from L1
• stale source descriptions do not leak into target description parts
• no __curation_merge__ sentinel is written
• LLM / Compactor / vector collaborators are fail-fast and must not be called
• replay after sources are already consumed is idempotent
• alias-layer failure before store writes leaves graph rows unchanged

validation

Local no-backend validation:

• uv run --extra test pytest tests/integration/compat/test_lineage_graph_compat.py --collect-only -q → 40 tests collected
• uv run --extra test pytest tests/integration/compat/test_lineage_graph_compat.py -q → 40 skipped (expected with no COMPAT_* backend env vars)
• uv run ruff check tests/integration/compat/test_lineage_graph_compat.py
• uvx ruff format --check ./aperag ./tests via pre-commit make lint

I also smoke-ran the new description-free path against InMemoryLineageGraphStore locally to confirm the target keeps {target, source} lineage, source is deleted, and source description text becomes "".

notes

This PR deliberately does not pull in Phase C score threshold / entity_type_alias work. It is the B2 contract layer only.