docs(readme): expand stub README to full structure (#46)

Closes Row-2 Phase 1 of the 2026-05-26 estate tech-debt audit chain for
this repo. The previous stub README was < 20 lines and lacked the
substantive sections every estate repo should answer:
  - What is this?
  - How do I use it (quickstart)?
  - What's in here (component layout)?
  - Status / maturity / licence.
  - Companion repositories.

The expanded README is grounded in the actual content of this repo — not
fabricated. Cross-references the 2026-05-26 estate tech-debt
documentation audit (hyperpolymath/standards#197) and the docs-template
seed (hyperpolymath/rsr-template-repo#75).

<!--
SPDX-License-Identifier: CC-BY-SA-4.0
Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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## Summary

<!-- What does this PR do, and why? -->

Closes #

## Type of change

- [ ] 🐛 Bug fix (non-breaking change that fixes an issue)
- [ ] ✨ New feature (non-breaking change that adds functionality)
- [ ] 💥 Breaking change (would change existing behaviour)
- [ ] 🕳️ Soundness fix (fixes a checker/proof false-negative)
- [ ] 📖 Documentation
- [ ] 🧹 Refactor / tech debt (behaviour-preserving)
- [ ] ⚡ Performance
- [ ] 🔧 Build / CI / tooling

## How has this been verified?

<!-- Establish ground truth: which tool did you RUN, and what did it
report?
     Don't cite a status doc — cite the command and its output. -->

## Checklist

- [ ] My commits are **signed** (`git commit -S`).
- [ ] I ran the project's own checks/tests locally and they pass.
- [ ] New files carry the correct `SPDX-License-Identifier` (code/config
`MPL-2.0`,
      prose `CC-BY-SA-4.0`); I did not relicense existing files.
- [ ] Docs are updated, and no public claim now overstates what the code
does.
- [ ] I have not introduced a soundness hole (or I have flagged where I
might have).

## Notes for reviewers

<!-- Anything that needs special attention, follow-up, or context. -->

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath