Adds a guide to compiling and evaluating a rule

[johnnyt]

Why

The package had no task page for the first thing a developer does with it:
compile a rule's text, keep the instruction list, and decide the rule for a
given context. The README's sections on compiling and evaluating are
reference by example; this adds the recipe.

What

• A how-to page, "How to compile and evaluate a rule", under docs/guides/.
  It states the goal and the starting point, then numbered steps in three
  stages (compile, keep the list, evaluate), each step with the result it
  shows. Its example is a library loan: a copy counts as lost when its loan
  is more than thirty days overdue and the copy is still checked out.
• One line in the README's Documentation map, under Do, linking the page by
  its absolute GitHub URL (docs/ is not in the published package's
  files, so a relative link would not resolve on npm).

No README text is replaced: the "Compiling a rule" and "Evaluating a rule"
sections stay as the reference the page links to. No file under src/
changes, and no changelog fragment is written (changelog.d/README.md
excludes documentation).

Notes

• Source material: the docs audit named no span for this subject, so the page
  is written from the package's own type declarations and the README's
  "Compiling a rule", "Evaluating a rule" and "The tagged subpath" sections.
• Every result the page states was checked against the source on this branch
  by running the page's snippets together (the instruction-list prefix, the
  JSON text, both decisions, the compile refusal's reason and position, and
  the unbound-variable refusal), and the snippets typecheck together under
  strict settings. The page is not executed by test/readme.test.ts, which
  reads the README only.
• The plain-JSON branch: a date literal and a float lose their type through
  JSON.stringify, and encodeTagged / decodeTagged round-trip the same
  list, which then evaluates; the page says so in one sentence and links the
  tagged subpath.
• README linter: README.md: 1168 lines, every part present.
• Gate: the full gate (mise exec -- pnpm run gate) ran green on this head;
  README.md is in gate.also_gated_paths, so the docs-only carve-out does
  not apply.
• Review: no review agents are declared for this repository; the author's
  own review round checked the page against the how-to rules (goal title,
  starting point, numbered steps each with a visible result, nothing taught
  beyond one sentence a step) and the repository's example-world rule.

Closes pts-3rn1