Validation policy: spec-aware check registry and planner

[zkwentz]

What & Why

Normalized subjects need a versioned policy to become an executable, auditable check list. This PR adds the check registry, source inspection, redaction, and capability-aware planner.

Risk & Review Guidance

• Risk level: MEDIUM — policy selection determines blocking behavior
• Task class: feature
• Review focus: src/openenv/validation/planner.py:227 for plan construction; src/openenv/validation/checks.py for criterion semantics; src/openenv/validation/serialization.py for report-safe evidence
• Blast radius: A planner error can omit a required criterion, assign the wrong severity, or leak submitted values into reports.

Decisions & Alternatives

• Policy owns severity and timeouts — rejected submission-controlled policy metadata.
• Unavailable capability becomes skip — preserves evidence that a check did not run.
• Centralize serialization/redaction — avoids each check inventing its own trust boundary.

Verification

• PASS: PYTHONPATH=src:envs .venv/bin/python -m pytest tests/ -q -m 'not integration' — 1,559 passed, 114 skipped, 31 external integration tests deselected on the complete stack.
• PASS: Fork whole-stack CI — 7/7 checks passed, including Python 3.11/3.12, lint, docs, lock validation, and package smoke tests.
• NOT verified: No official remote-runner policy calibration was possible.

Scope & Non-Goals

• This layer builds plans; it does not execute checks or start environments.

Stack

PR 3 of 17; depends on #943.

Full 17-PR stack

1. #942 RFC 008: Spec-neutral local and remote validation architecture
2. #943 Validation foundation: spec-neutral contracts and adapters
3. #944 Validation policy: spec-aware check registry and planner ← current
4. #945 Validation executor: hardened shared reports and eligibility
5. #946 Local validation API: execution-model-aware profiles
6. #947 CLI: validation profiles and shared reports
7. #948 HF runtime: dedicated sandbox execution mode
8. #949 HF certification: enforce trusted dedicated isolation
9. #950 [RFC 008] Define remote author validation and publish gating
10. #951 Validation: unify CLI reports and add a strict publish profile
11. #952 Validation: add structured author diagnostics and remediation
12. #953 Validation: run author validation in dedicated HF Sandboxes
13. #954 Validation: isolate submitted code in author Sandboxes
14. #955 Validation: bind author reports to policy and source
15. #956 CLI: scaffold publish requirements
16. #957 CLI: gate pushes on staged remote validation
17. #958 Docs: explain local, remote, and publish validation

All upstream PRs remain draft while RFC 008 is under review.