feat(abtest): hypothesis metadata for pre-registration (Change Set A)

[imjlk]

Summary

Change Set A from ABTEST_ADVANCED_EXPERIMENTATION_FOLLOWUP.md — hypothesis pre-registration for A/B tests.

What changed

• HypothesisMetadata domain module (hypothesis.ts): structured objective, primary metric, expected lift (relative/absolute), owner, experiment scope (family key, attribution/exclusion windows). Canonical SHA-256 checksum + locking for pre-registration integrity.
• AbTest type extensions: optional hypothesis and assignmentProvenance fields.
• Schema/validator: assignmentProvenance in abTestSchema and persistence validator.
• Graph anchor: hypothesis tests (197 paths).

Not in this PR

• Stratified split, collision guard, preview/seed send, decision snapshots — subsequent change sets B-E.
• Hypothesis wiring into createTest/launchAbTest input — next commit.

Checklist

• format, check (197 paths), build, test (200 abtest + 46 full)
• Sampo changeset (@listmonk-ops/abtest minor)

Reviewer notes

• @codex — please review the HypothesisMetadata type design, checksum canonicalization, validation rules (strict vs draft), and experimentFamilyKey format.

Summary by CodeRabbit

• New Features
  • Added optional pre-registered A/B test hypothesis metadata with deterministic checksum locking and verification.
  • Added assignment provenance to reflect whether assignments are manifest-bound or unavailable.
  • Added opt-in recipient-domain/provider stratification to compute and persist a constrained quota matrix for holdout.
  • Updated the CLI (including interactive flow) to accept --hypothesis and --enable-stratification.
• Bug Fixes
  • Strengthened load-time validation for persisted hypothesis and stratification data, including checksum tamper detection.
• Documentation
  • Expanded A/B test docs (EN/KO) covering hypothesis locking and stratification usage.
• Compatibility
  • Existing A/B tests continue to work since all additions are optional.