test: consume conformance API contract fixture

[rlippmann]

Summary

• sync conformance fixtures from Python source including tests/fixtures/conformance/api/public-api-v1.json
• add TS consumer test for the API contract fixture
• verify required top-level exports and engine members via explicit Python-to-TS mapping
• add transcript type aliases and exports (TranscriptMessage, Transcript, ApplyResult) to satisfy contract parity

Why

Python now publishes a conformance API contract fixture; consuming it in TS makes API surface parity explicit and regression-tested.

Scope checklist

• Diff is minimal and scoped to the requested behavior
• Python/spec parity is preserved (or mismatch is explicitly documented)
• Fixture semantics are respected (exact string vs non-empty null rule)
• No unrelated refactors
• Relevant tests were run

Notes

• Validation run: npm test
• Validation run: FIXTURES_SOURCE=/Users/rlippmann/Source/context-compiler/tests/fixtures/conformance npm run fixtures:check
• Preprocessor API contract fixture is not consumed in TS tests in this PR.