refactor: remove transcript replay APIs

[rlippmann]

What changed

• Removed transcript replay from the Python public API:
  • compile_transcript(...)
  • Engine.apply_transcript(...)
  • Transcript
  • TranscriptMessage
  • ApplyResult
• Removed transcript replay tests, conformance fixtures, API fixture entries, and package exports.
• Replaced the transcript replay example with an example demonstrating explicit Engine.step(...) sequencing and checkpoint restore.
• Updated Demo 6 to use explicit sequencing instead of replay.
• Removed replay references from the README, API reference, architecture docs, and related documentation.
• Added focused parser regression tests identified during the replay-removal audit to maintain coverage.

Why

Transcript replay no longer reflects the preferred authority-layer architecture.

The supported workflows are now:

• Engine.step(...)
• state import/export
• checkpoints
• controller APIs

Removing transcript replay before synchronizing the TypeScript implementation and before releasing context-compiler-example-integrations simplifies the public surface and avoids carrying forward an API that is no longer recommended.

This is an intentional pre-1.0 public API contraction.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)