Clarify semantic record contracts and multi-contract types

[callumalpass]

Summary

• define record contracts as compact application-facing semantics rather than storage or wire formats
• clarify binding policy and direct non-transforming field mappings
• add shared conformance coverage and a neutral testbed scenario for one type implementing multiple contracts

Validation

• python3 scripts/check_test_levels.py
• python3 scripts/check_v03_tests.py
• python3 scripts/prototype_tasknotes_v03_migration.py --check-fixture
• npm test --prefix packages/testbed
• npm run build --prefix site
• mdbase TypeScript v0.3 conformance: 124 passed
• mdbase Rust v0.3 conformance: 6 suites passed (with the companion count update)