docs: stop using -- as a dash, and fail CI on it

[TMHSDigital]

-- belongs to command lines (flags, git's end-of-options separator) and nowhere else. It had been standing in for a dash in 26 places. Each now uses a comma, colon or parentheses, whichever the sentence wanted.

Closes #54.

What changed

• Prose:
  • METHODOLOGY: 10 places.
  • Docstrings and comments in config.py, loader.py, baseline.py, markdown.py and types.py.
  • conftest.py, one test, and one example.
• One report string: the load summary's note on ordinal-score rows now reads "...has no ordinal support: flattening levels into unordered options discards the ordering, so they are loaded...". docs/example-report.md carries that note, so it changed with it. The site build compares the report line for line with what its command prints, and it still passes. This changes report text, not any number.
• The site smoke test's failure marker was also a --. It is now FAIL.
• CI: a second step in the prose job fails on a bare -- between words, or one opening a continuation line. It scans the markdown, src/, tests/, examples/, conftest.py, site/ and scripts/.
  • Flags (--adapter) have no space after the dashes, and HTML comments (<!--) and table rules (|--:|) have no word before them, so none of these trip it.
  • On main it finds all 26. On this branch it finds none. A probe of real command lines and an HTML comment matched nothing.
• Kept, because the syntax needs it: git's -- separator in build_site.py and in the workflow.

Checked: ruff, mypy --strict, and pytest pass. The site build (including the example-report comparison), check_floor_parity, check_site_links, and smoke_site all pass. The em-dash check is also clean.

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