feat(policy): add manifest validation rules and canonical test fixtures

[rosspeili]

Summary

Add deterministic validation rules and fixture sets for policy manifests/envelopes.

Why

Tooling, tests, and contracts need shared validation expectations.

Scope

• Validate basis points sum constraints
• Validate beneficiary/cohort references
• Add valid/invalid fixture corpus

Acceptance Criteria

• Validation outcomes deterministic and test-covered
• Fixtures reusable by tests and future SDK