[AZ-220] Add shared runtime contract models

Implement the shared DTO contract surface with validation so runtime components consume one public model set instead of duplicating shapes.

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/03_implementation/reviews/batch_02_review.md

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+# Code Review Report
+
+**Batch**: AZ-220_shared_runtime_contracts
+**Date**: 2026-05-03
+**Verdict**: PASS
+
+## Findings
+
+| # | Severity | Category | File:Line | Title |
+|---|----------|----------|-----------|-------|
+| - | - | - | - | No findings |
+
+## Review Notes
+
+- AC-1 is satisfied by the public exports in `src/shared/contracts/__init__.py` and the DTO models in `src/shared/contracts/models.py`.
+- AC-2 is satisfied by Pydantic validation for missing required fields, raw frame retention, optimistic covariance reporting, and inconsistent anchor decisions.
+- The implementation stays inside `shared/contracts` ownership and does not introduce component-specific algorithms.
+- Raw frame payloads remain references only; the model rejects retained raw-frame payload flags.
+
+## Verification
+
+- `.venv/bin/python -m black --check src tests e2e/replay` passed.
+- `.venv/bin/python -m ruff check src tests e2e/replay` passed.
+- `.venv/bin/python -m pytest` passed: 11 tests.