feat(core): add runtime validation helpers

[salim4n]

Summary

• add typed runtime assertions and non-throwing validators for core datasets, variants, adapters, run results, usage metrics, traces, metric/reward results, normalized scores, and JSON-compatible fields
• wire createDataset, createMockAdapter, and normalizeRunResult through compatible validation paths
• update core docs plus roadmap/readiness/audit docs for PR feat(core): add runtime validation helpers #48 and the next planned PR sequence

Validation

• bun run lint
• bun run typecheck
• bun test packages/core/src/index.test.ts
• bun run test
• bun run build
• bun run pack:check
• git diff --check