Add typed execution models and schemas for solver-agnostic control plane foundation

[fraware]

Summary

• Add Pydantic execution-plane models (VerificationObligation, routing, backend execution, coverage) with supporting types
• Add canonical digests (obligation_id, routing_id, abstraction/payload/attempt digests) and material URI validation
• Ship JSON schemas for obligation, routing, execution, and coverage; update SCHEMA_INDEX
• Add serialization round-trip, determinism, and schema validation tests
• No adapter runtime, evaluate_lane, or router selection behavior changes

Test plan

• pytest tests/test_execution_models.py -v
• Confirm no changes to adapter_runtime / evaluate_lane / router selection logic

[fraware]

Superseded by the full control-plane integration PR from feature/solver-agnostic-control-plane (includes sprint 1 plus sprints 2–13e, holdout, and consumer gate docs).