Validation foundation: spec-neutral contracts and adapters

[zkwentz]

What & Why

The RFC needs a concrete extension seam that can describe served, one-shot, and external validation subjects without importing submitted code. This PR adds the spec-neutral contracts, registry, served OpenEnv adapter, and optional Harbor requirements adapter.

Risk & Review Guidance

• Risk level: MEDIUM — new foundational public-internal contracts and parsers
• Task class: feature
• Review focus: src/openenv/validation/specs/base.py:490 for adapter detection/loading; src/openenv/validation/specs/harbor.py:397 for requirement normalization; src/openenv/validation/models.py:343 for the check contract
• Blast radius: Bad detection or normalization would select the wrong policy or misstate resource/network requirements.

Decisions & Alternatives

• Signature-only detection — adapters inspect structure and never import submitted code.
• Reject zero or ambiguous matches — rejected registry-order fallback because it is non-deterministic and unsafe.
• Keep Harbor auxiliary — it supplies requirements to the served OpenEnv subject rather than becoming a core dependency.

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 third-party spec adapter was available for end-to-end validation.

Scope & Non-Goals

• No public plugin discovery and no one-shot task command; those remain follow-up work for Spec-agnostic task validation: openenv validate-task for external task.md packages (PostTrain Arena / BenchFlow) #898.

Stack

PR 2 of 17; depends on #942.

Full 17-PR stack

1. #942 RFC 008: Spec-neutral local and remote validation architecture
2. #943 Validation foundation: spec-neutral contracts and adapters ← current
3. #944 Validation policy: spec-aware check registry and planner
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.