Fix the equation rendering, and document the coupling with tutorials

[timlichtenberg]

What this does

Closes #15.

The equations were rendering twice, once typeset and once as a flattened, mis-ordered copy. The cause was not the dollar signs the issue suspected: the documentation loaded KaTeX's script without katex.min.css. That stylesheet is what hides the MathML KaTeX emits next to its own HTML output, so without it the browser rendered both. The documentation now typesets with MathJax 3, the renderer PROTEUS, ZEPHYRUS, JANUS and MORS already use, which needs no stylesheet of its own.

A second defect turned up on the same page: the seven-item list of what an impact does was breaking after item five, because the paragraph continuing that item was not indented, so items six and seven printed as running prose.

What else is in here

A checker. tools/check_docs_render.py compares each built page against its source and fails on a list that lost items, on TeX that reached the page as text, and on a page whose maths renderer cannot style what it produces. It is validated in both directions: against the build before this branch it reports exactly the defects above, and against the build after it reports none. It reads the built HTML rather than running the page, so it catches a renderer configured wrongly but not one that fails to load, and it says so.

Two tutorials. One runs a system of six embryos standalone, from a settings file to a figure of the tracks; the other is a complete PROTEUS configuration that grows a planet by giant impacts with every other module set to dummy. Both were executed rather than written from memory: every number, every log line and the figure come from those runs, and the PROTEUS configuration loads through the configuration validator.

The other two accretion modules get a page each. timeline replays an impact history from a file, which is how a Morrigan-derived history is reproduced, shared, or handed to someone without Morrigan installed; dummy builds one from scaling laws. Both are PROTEUS modules rather than Morrigan ones, documented here because they occupy the same slot and because a Morrigan run produces exactly the file timeline consumes.

The coupling pages gain what the framework does with a schedule once it has one: it is written to the run's output directory, a resumed run replays that file, and the same file can be replayed on its own. The offset is already applied when the file is written, which is worth knowing before replaying it.

Presentation. The framework page shows the current PROTEUS lockup rather than the previous logo, and now also the code architecture, where the accretion module and the loop stage that applies an impact are visible. Both PROTEUS figures are loaded from the PROTEUS repository rather than copied into this one. The front page lays its links out as cards, matching the other module documentation, and the contact address is the current one.

Validation

• zensical build --clean is warning-free; tools/check_docs_render.py reports no problems.
• Every internal link and anchor resolves, and every link to another ecosystem site returns 200.
• Both tutorials were run end to end, twice for the standalone one to confirm a fixed seed reproduces it exactly.
• Three independent review passes over this branch; every confirmed finding is fixed or recorded.

One thing to know before merging

docs/proteus-framework.md pins the four PROTEUS figure URLs to the tl/accretion-morrigan-coupling branch, because main has accretion in neither figure yet. They return to @main once PROTEUS#789 is merged, and the checker prints the outstanding pin on every build until then.