test(structural): add parser failure golden fixture

[nia-sg-bot]

Summary

• add a checked-in golden artifact for a malformed Python snapshot
• make the parser-failure regression compare the schema-valid, scoped warning artifact against that fixture
• lock the deterministic no-topology outcome for syntax errors: no symbols or relationships, with an explicit PARSE_FAILURE warning

Why this slice

Issue #22 requires golden coverage for parser failures and guarantees that parse failures become scoped warnings rather than silent empty snapshots. This narrow fixture makes that behavior reviewable and protects the deterministic Tree-sitter baseline without changing the public schema or CLI contract.

Compatibility

• no artifact schema, CLI behavior, or extension-facing contract changes
• the fixture asserts the existing schema_version: 2.0 structural output subset and warning semantics

Validation

• python3 -m pytest -q — 150 passed
• python3 -m compileall -q diffgraph tests
• git diff --check origin/main...HEAD
• python3 -m diffgraph.cli --help

Part of #22

Remaining work

Issue #22 remains open for the broader deterministic Tree-sitter baseline and its remaining fixtures.

[coderabbitai]

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[nia-sg-bot]

This PR appears to be a duplicate of already-merged PR #43 (test(structural): add parser failure golden fixture). The changes from PR #43 were merged to main on 2026-09-01T20:02:43Z by Avikalp. Closing as no further action needed.