test: fail when docs/ serves a stale notebook

[project-delphi]

Follow-up to #57, which is where this came up: docs/notebooks/ had gone stale against notebooks/ again, and nothing in CI said so.

The gap

notebooks/*.ipynb are resources: in _quarto.yml, not render: targets, so Quarto copies them into docs/ verbatim. That puts them in the blind spot between both existing gates:

• the regenerate step reruns gen_notebooks.py and fails only if the tracked notebooks/ drift from the normalizer's output — it never looks in docs/;
• compare_render.py walks *.html and skips everything else.

So a notebook committed without a re-render leaves Pages serving the old copy with every check green. It has now happened twice, once to nine of the twelve notebooks at a stroke.

The check

New check 2, docs/notebooks matches notebooks/: each committed notebook compared byte-for-byte against the copy docs/ serves, plus orphans in docs/notebooks/ that a rename left behind.

Byte-exact is the right comparison here for the reason the HTML gate cannot be: Quarto copies these files rather than transforming them, so there is no SCSS-style platform difference to normalize away.

The workflow already runs check_links.py against docs/ as committed, before it re-renders — the copy Pages is serving — so the gate lands in the right place for free.

--notebooks-only is unchanged and still numbers its two checks 1 and 2; this one reads docs/, so it belongs with the site checks. Kahoot's TODO-only check moves from 9 to 10; CLAUDE.md is updated for both that and the count.

Verified

• Passes on current main: 14 notebooks served from docs/ are byte-identical to the committed notebooks/.
• Fails as intended when they diverge — edited one markdown cell in notebooks/02-thinking-in-n-dimensions.ipynb without re-rendering:

  [2] docs/notebooks matches notebooks/
    FAIL  docs/ serves an old copy of 1 notebook(s): 02-thinking-in-n-dimensions.ipynb — run `quarto render` and commit docs/
  

  Exit code 1. The edit was reverted; it is not in this branch.
• Both check_links.py and check_links.py --notebooks-only pass, with the numbering right in each mode.

🤖 Generated with Claude Code

https://claude.ai/code/session_0125vs3ARnz2pLp8D5vQ6iKJ