Finish draft: 'When do we need a chain?'

`chapter-04-architecture/01_when-do-we-need-a-chain.md` is a skeleton. Typos corrected, but content is missing:

- [ ] The CAP-theorem sentence trails off ("...we can either have consistency, or...") — complete it (or defer to the dedicated CAP page `08_cap-theorem.md` and cross-link)
- [ ] The "Read vs Write" section is an empty heading — write it
- [ ] Flesh out the on-chain/off-chain decision discussion

Tracked in TODO.md.

Start with chapter-04-architecture/01_when-do-we-need-a-chain.md and compare the incomplete CAP sentence with 08_cap-theorem.md. Read TODO.md for the surrounding scope, then complete the “Read vs Write” section and the on-chain/off-chain discussion. Done means the skeleton is filled, the CAP explanation is complete or cross-linked, and all listed TODOs are resolved.

Written by the indexing model from the issue text.