Rewrites the README as an introduction

[johnnyt]

The README becomes an introduction and a map: a What under the title, a Why, Install, one Basic usage example, a Documentation section grouped by the reader's question (Learn, Do, Look up, Understand) and Compatibility. It ends at 117 lines, down from 414.

What changed

• What (two sentences under the title): the shared type registry the family's documents, blocks and expressions agree on, and the questions it settles from a document alone.
• Why this package: the problem is two tools (the block editor's read check and the expression editor's path and value offers) reading one host document their own way and drifting apart; the difference is one pure reader, depending on nothing else in the family, that both take. No history and no predecessor.
• Install: unchanged pin, {:statifier_datamodel, "~> 0.5.0"}. The repo's release extension (.claude/wurk/release.md, "The README install pin: write the .0 form") requires the exact-minor .0 form, and that is the current minor.
• Basic usage: one fenced iex> block (26 lines) in the library loan: a document with a library.loan record and a Renewable shape, indexed, the type at a path read out and handed to the read check, which names the optional renewals field as missing, and an undeclared path answering :unknown.
• Documentation: Learn links the basic usage; Do links the Compatibility, Coverage and Schema module pages, each saying it is the API reference until a guide page exists; Look up links the Index, Document, Declarations and Types module pages and the changelog; Understand links the root module page (what is here and what is deliberately elsewhere) and the decision records on GitHub.
• Compatibility: the Elixir requirement from mix.exs, no runtime dependencies, and the pre-1.0 policy that used to sit as a banner above the introduction.
• test/readme_test.exs still runs doctest_file("README.md"); only its moduledoc changes, since the README is no longer the per-function reference.

Unchanged except: the H2s Installation (now Install), What it answers, What is not here and Who takes this are removed; Why this package, Basic usage, Documentation and Compatibility are added; License stays. The removed sections' substance is already in the module documentation the map links: the per-function walk in each module's moduledoc, "What is not here" in the root module's moduledoc, and "Who takes this" is covered by the Why. The card-processing examples go with them, so no example in a fixture-only domain remains in the README.

Checks

• mix quality (full) green on this tree: format, compile, credo, docs, doc links, dependencies, dialyzer, 137 of 137 tests at 99.1% coverage. The commit is the tree that run read, with the gate lock and a machine slot held across both.
• Two sabotage checks on the new example, each restored byte-equal: changing the read check's expected {:missing, ["renewals"]} and the final expected :unknown each turned doctest README.md (1) red on the assertion.
• readme-lint.rb README.md --manifest .claude/diataxis.md: README.md: 117 lines, every part present (exit 0).
• readme-lint.rb README.md (no manifest): README.md: 117 lines, every part present (exit 0).
• No file under lib/ changes.

The README stage flip

ex_quality 0.16.0 is not on Hex (latest 0.15.1), so this PR takes the acceptance's fallback: the ex_quality dev dependency stays at ~> 0.15 and .quality.exs is not edited. Enabling the README stage at error is left for a follow-up once 0.16.0 is published.

Provenance

• The fallback above is the acceptance's own clause, decided by the conductor under a standing consent, 2026-10-05.
• The install pin keeps the .0 form the release extension prescribes rather than the bare ~> 0.5 form.

Review

In-turn review of the text (this tier takes no conductor-dispatched code review): every Documentation link lands on a module page or a GitHub path that exists; the Do links say they point at the API reference until a guide page exists; no divider heading reclassifies sections; the What's claims ("what the document alone can settle") match the root module's moduledoc; both packages the Why names depend on this one on their main branches; the introduction carries no contributor-only text and no avoided term.