feat(e4): add architecture guardrails and regression gates

[agustif]

Summary

Implements Epic #210 and sub-issues #222 #223 #224 #225.

This PR adds architecture guardrails and regression gates to prevent boundary drift, import-cycle growth, packet mode drift, and persistence safety regressions.

Scope

E4.1 Import boundary checker

• Added new import-linter contract in .github/importlinter.ini:
  • [importlinter:contract:base_no_upward_imports]
  • Enforces: desloppify.base must not directly import app|engine|intelligence|languages
  • Uses explicit legacy ignore_imports entries to lock current debt while preventing new violations.

E4.2 Cycle detection gate

• Added cycle checker script:
  • .github/scripts/check_import_cycles.py
• Added scoped SCC allowlist:
  • .github/architecture/cycle_allowlist.txt
• Wired into make arch in Makefile so CI fails on new scoped SCCs in:
  • desloppify.app.commands.review
  • desloppify.engine._plan
  • desloppify.engine._state
  • desloppify.engine._scoring

E4.3 Packet parity golden tests

• Added golden fixture:
  • desloppify/tests/review/policy/golden/review_packet_shared_fields.json
• Added golden regression test:
  • desloppify/tests/review/policy/test_packet_mode_parity_golden.py
• Keeps parity lock across prepare/run-batches/external for shared packet semantics.

E4.4 Persistence safety property tests

• Added randomized regression/property-style tests:
  • desloppify/tests/plan/test_persistence_safety_properties.py
  • desloppify/tests/state/test_persistence_safety_properties.py
• Verifies malformed/newer payload handling and unsafe-save blocking behavior stays fail-loud.

CI contracts and docs

• Extended CI contract tests:
  • desloppify/tests/ci/test_ci_contracts.py
• Added architecture guardrail docs:
  • docs/architecture_guardrails.md

Validation

1. Architecture gate

• source .venv/bin/activate && make arch
• Result: pass (import contracts + cycle checker)

2. CI contracts

• source .venv/bin/activate && make ci-contracts
• Result: 11 passed

3. Regression suites

• source .venv/bin/activate && pytest -q desloppify/tests/ci/test_ci_contracts.py desloppify/tests/review/policy/test_packet_mode_parity.py desloppify/tests/review/policy/test_packet_mode_parity_golden.py desloppify/tests/plan/test_persistence_safety.py desloppify/tests/plan/test_persistence_safety_properties.py desloppify/tests/state/test_state.py desloppify/tests/state/test_persistence_safety_properties.py
• Result: 73 passed, 1 skipped

Stack Context

This branch is stacked on implementation work from #227 (E2) and #228 (E3). Guardrail tests/gates are designed to enforce those behaviors going forward.

Tracking

Parent program: #206