docs: rewrite README.md sentence by sentence against the prose rules (closes #672)
Rewrites README.md from scratch for #672: what the program is, how to use it, every setting, the design, then development and tests. Every statement was checked against the code on next. Where the old text and the code disagreed, the code decides; the PR body lists each disagreement and judgement call. Model: fable-5-1
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