Adds an explanation of why the language is its own

[johnnyt]

Adds an explanation page, "Why a predicate language of its own", and wires it into hexdocs and the README.

What changes

• docs/explanation/why-a-predicate-language-of-its-own.md (new): who writes a rule and who executes it, the alternatives turned down (every rule written in code; evaluating the text as Elixir; a tree walker that calls host functions by name; a rule format of the application's own), what a language of its own buys, why it compiles to instructions rather than walking a tree, and what it costs. Examples are set in the library loan (renewals, holds, overdue copies). The page has no numbered steps, no instructions and no code blocks, so nothing on it needs executing to read it.
• mix.exs: the page is an extra, sits in the existing Explanation group after the architecture page, and is listed in package.files (the README links it relatively).
• README.md: one line under Understand in the Documentation map.

No file under lib/ changes. No changelog fragment: changelog.d/README.md excludes documentation.

Source material

The page started from the README's "Why a predicate language of its own" paragraph and was written out from the package's own records: the no-eval decision record (ADR-0004) for the threat model and the turned-down implementations, ADR-0001 for the compile-versus-tree-walker trade, ADR-0017 for the rule-format alternative, and the architecture page's design decisions. The docs audit's per-file next actions name no record for this subject.

The README's Why paragraph stays where it is: the README layout requires a Why section (the README check reports no_why without one), so the page deepens the paragraph rather than replacing it. No README text is replaced.

Claims checked against the library at this branch: a missing value returns an UndefinedVariableError; an unregistered function name returns an "Unknown function" error value; a raising host function is rescued into an error value (the evaluator's function dispatch); renewals < 3 AND NOT on_hold with renewals at 3 returns false without reading on_hold.

Checks

• Full gate (mix quality) green: mix.exs is a build path, so the docs-only exemption did not apply. Doc links: 54 links checked, 0 findings; Docs: no warnings.
• mix docs --warnings-as-errors clean; the rendered page sits in the Explanation sidebar group and its relative links resolve to the rendered architecture, custom functions, embedding, simple subset and language pages.
• README check: README.md: 113 lines, every part present.
• The page carries no result lines, so none is unexecuted.
• "run" as a noun on the page and in this body: none. Fixture-only domain words on the page and in this body: none.

Provenance

The title is the bead's as given; an explanation title needs no "How to" prefix.

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