v2 Stage 2: L2 (callee backfill + call_graph re-identity)

[rahlk]

Part of #69.

Stage 2 — L2 (callee backfill + call_graph re-identity)

Branch: feat/v2-stage2-l2

Deliverables:

• backfill_callees(app, sig_to_id) — at -a 2, set each call body node's callee to the resolved can:// id (the one sanctioned null→id refinement).
• Re-identify call_graph edge src/dst to can:// ids; externals keep their external ids.
• Neo4j PY_CALLS endpoints re-keyed via sig_to_id.
• Superset test L1 ⊆ L2.

Files: core.py, neo4j/project.py, test/test_v2_l2.py, extend test/test_v2_conformance.py.

Gate: no dangling call_graph endpoints (every endpoint resolves to a callable/external id); assert_conformant(..., 2); L1 ⊆ L2 holds.

Expand into full bite-sized tasks with superpowers:writing-plans at stage start (against the realized Stage 1 APIs).

[rahlk]

Deferred from Stage 1 (Task 1.9 Neo4j L1 re-key): now that PySymbol/PyModule nodes key on the can:// id, the rows.py::edge_to_symbol deferred edges resolve targets by signature and therefore drop silently at finish() for in-project targets. Stage 2 must re-route them to can:// ids (thread sig_to_id through edge_to_symbol: declared → ("PySymbol", id), else external):

• PY_RESOLVES_TO (callsite → callee) — partly shadowed by the callable-level PY_CALLS twin.
• PY_EXTENDS (class → base) — no twin; pure L1 hierarchy data loss — must be handled explicitly.

Also reconcile test/test_neo4j_schema.py: 6 stale 2-arg project() calls, and test_all_catalog_node_kinds_and_relationships_are_exercised will go red on the missing edges once its pre-existing PyCFG import error is repaired.