fix: add missing apply_transcript parity

[rlippmann]

Summary

• add engine.apply_transcript to the TS engine surface and implement replay-on-current-state behavior
• keep compile_transcript replay semantics aligned by delegating through apply_transcript
• add focused transcript parity tests only for API-level coverage not currently covered by TS conformance fixture consumption

Why

Python has exposed engine.apply_transcript(...) since v0.3.0, so this fills a missing TS parity gap rather than adding a new API concept.

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 (all passing).
• No conformance fixture files were modified.