Make JSON HTTP API reference path-first

[danielporterda]

Summary

• group the JSON HTTP API table of contents by path instead of by verb-prefixed operation
• render path-only endpoint headings with inner per-method verb blocks while preserving method-specific anchors
• refresh the published OpenAPI characterization output and regression tests for the new structure

Validation

• PYTHONPATH=src /Users/danielporter/control/x2mdx/.venv/bin/python -m pytest tests/test_openapi.py tests/test_openapi_published_fixtures.py -q
• PYTHONPATH=src /Users/danielporter/control/x2mdx/.venv/bin/python -m pytest tests/test_characterization_outputs.py -q
• PYTHONPATH=src /Users/danielporter/control/x2mdx/.venv/bin/python -m pytest -q