Graph-level roundtrip invariant test for reconstruct path

[pedroanisio]

Context

codebase_mapper/reconstruct.py (136 lines) materializes a working tree from inventory.ttl + the blobs/ directory. We have tests/verify_roundtrip.py for byte-identical blob-based roundtrip, but no invariant test that asserts the round-trip preserves the RDF graph itself.

The code-mapper bundle (analyzed in this session) carries tests/test_python_parser.py::TestRoundTrip.test_roundtrip_preserves_structure — exactly this shape: parse → re-serialize → re-parse → assert structural equality.

PALS's law (per CLAUDE.md): absence of an invariant test for the reconstruct path is an architectural omission.

Scope

Add tests/verify_reconstruct_invariant.py:

1. Run the pipeline against a small fixture repo (existing static/schemas/ works as a low-content target, or a synthetic 2-file Python repo).
2. Capture the emitted inventory.ttl.
3. Reconstruct the working tree via codebase_mapper.reconstruct.
4. Re-run the pipeline against the reconstructed tree.
5. Assert the two RDF graphs are isomorphic (or hash-equal under canonical N-Triples sorting).

Implementation notes

• Use rdflib.compare.isomorphic for the graph-equality check — handles blank-node renaming.
• Or simpler: sort N-Triples lines after canonicalization and compare line-by-line.
• The test should be fast (<5 seconds) — keep the fixture tiny.

Failure modes to catch

• Reconstruct loses a predicate that was emitted by some plugin.
• Reconstruct re-emits a predicate with a different blank-node ID that doesn't canonicalize.
• The blob-content path round-trips bytes but a metadata predicate (e.g. cbm:size) is computed differently the second time.

Acceptance

The verifier passes today (or fails loudly with a diff of triples that differ). Then add it to the README ## Verify section's run list.

Related

• tests/verify_roundtrip.py (existing) — byte-level
• tests/verify_regenerate.py (existing) — TTL+AST regeneration semantic vs byte fidelity
• This issue: graph-level invariant — the third corner of the triangle