Frozen renders, and execute settings for projects #5

A screen reader of a book (marginalia) must show a chapter's code outputs and figures without ever running its code. scriptorium caches every run in .scriptorium/freeze.json, but a render either executes (and runs on a miss) or does not execute (and shows no outputs). Wanted: a frozen render that serves cache hits and never starts an interpreter; a miss shows the source and warns.

Separately, execute: settings never reach a project: project.load forwards bibliography/glossary/… but not execute, so a book cannot set interpreters, and the 30 s run timeout is fixed. Training a small model in a book chapter takes longer.

Lands on the 2-render-html branch (#3), since its consumer is the same marginalia change.