test: add structured regression fixture harness parity

[rlippmann]

Summary

• import Python structured regression fixtures under tests/fixtures/engine-regression/structured
• add a TS harness that consumes Python scenario/expected format directly
• compare per-turn input, decision.kind, decision.prompt_to_user, and full checkpoint artifacts
• add structured fixture sync/check scripts and npm script entries
• align clarify prompt strings where structured fixtures exposed true parity mismatches

Why

This extends TS parity coverage from conformance step/transcript fixtures to Python structured per-turn regression fixtures, including checkpoint artifacts.

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 (passing)
• No preprocessor fixture consumption changes
• No CI workflow restructuring