RESULT: SUCCESS

FILES CHANGED:
- cmark
- parser/blocks.py
- parser/__init__.py
- parser/inlines.py
- renderer/html.py
- test_cli.py

SUMMARY:
Implemented standard-input/output CLI handling, UTF-8 validation, deterministic stderr error reporting, heading and paragraph separation, and CLI regression tests.

Verification: 8 tests passed.

BLOCKERS:
- None