R1-004: Test compatibility and durable core paths

[1sgtpepper]

Summary

Adds one executable architecture contract test that runs the public compatibility path and the durable frontend/query/engine path over the same documents. It proves exact diagnostic equivalence, stable output under shuffled-equivalent input, the public AnalysisResult summary, and distinct clean, boundary, and violating cases.

Linked issue

Fixes #137.

PR Checklist

• Linked issue checked.
• One layer or one mechanical change.
• Behavior/schema/golden impact checked.
• Tests/docs updated.
• Local checks run.
• Keep draft until required CI is green.

Release target

R1 foundations

Single layer changed

• Test-only architecture conformance evidence

Behavior impact

• No user-visible behavior change

Tests and docs

• Tests added/updated
• No documentation change required; the architecture terms and ownership remain unchanged

Dependency checklist

• Checked docs/contributing/pr-dependency-checks.md and updated every dependent artifact.

Local checks

pytest tests/test_architecture_contracts.py::test_pure_core_layers_execute_through_compatibility_shell_and_kernel -q
pytest -q
ruff format --check .
ruff check .
pyright
lint-imports --no-cache
python tools/architecture/terminology_drift.py --format json
mkdocs build --strict
git diff --check

All passed; the focused test passed and the full suite reported 313 passing tests. Deliberately weakening the direct-versus-compatibility equality assertion is the negative control for path drift.