Write a repository README

Write a README for this project.

Order, strictly:
1. **One sentence: what this is and who it is for.** Not a tagline.
2. **Quickstart** — the shortest path from clone to something working. Exact commands. *If this takes more than about ten minutes or five commands, say so and say why; that is the finding.*
3. What you should see when it worked.
4. Prerequisites, but only the ones that actually block. Move the rest down.
5. Common failures during setup, with the fix. Every project has two or three and they are almost never written down.
6. Configuration — only what must be set. Defaults stay in a table further down.
7. How to run the tests.
8. Everything else — architecture, contributing, license.

Rules:
- *Commands must be copy-pasteable.* Where a value has to be supplied, say where to obtain it.
- Do not open with badges, a logo, or a feature list. Someone reading this wants it running.
- **Where I did not tell you how something works, write `TODO:` with the specific question** rather than a plausible instruction. A wrong setup step costs more than a missing one.
- State what this project does *not* do, if there is an obvious adjacent thing people will assume.