Finalize the Mithril SNARK Book

## Why

The SNARK book grew ticket by ticket: most topics sit flat under `Design Topics`, ordered by the issue that opened them. Specifications, decisions, benchmarks, runbooks, and transient plans look identical on the page, so a reader cannot tell what is still true, and durable content (error variants, type aliases, DST alignment) is buried inside closed tickets.

## What

Restructure the book along the proposed lifecycle and layer axes, fix the recorded inconsistencies, and produce the final document.

## How

- [ ] Apply the structure proposal to the book
- [ ] Deal with the inconsistencies listed in the analysis (notation, DST names, tier names, duplicated error lists)
- [ ] Move the durable content out of the closed tickets and archive the transient pages
- [ ] Write the content (concise)
- [ ] Review the final document