Runestone constructs: harmonize sample book, development schema, and code

[rbeezer]

The sample book's Runestone chapters produced 217 errors against the
development schema. A three-way audit — what the samples author, what
the XSL actually reads (the interactive builders, the static
representations, and the assembly's classifier, which stamps
exercise, the project-like blocks, and task alike), and what the
schema models — showed nearly all of it was the schema trailing the
implemented feature set, with a handful of genuine sample defects.
This branch closes the gap from all three sides. After it: the sample
book validates with exactly one error (a @reduce placement question
parked for #3047 or the FITB maintainer), and the sample article's
count drops from eleven to zero.

The commits:

• 72bfa84 (HTML) Authored slate markup is copied by local name
  only. An HTML page is this content's sole destination, and HTML
  parsers have no notion of namespaces — so the XHTML namespace
  becomes purely a source-side validation marker, declared once (a
  prefix on the slate), with plain HTML tags in the output. The
  mode (copy-authored-xml) is not slate-specific and could serve
  other embedded markup.

• 9a8a299 (Sample book) Four conformance repairs (a bare-text
  statement, a p wrapping an image, a stray p inside a listing,
  a trailing paragraph now a proper conclusion), and the show-eval
  demonstration declares xmlns:xhtml once on its slate, authoring
  xhtml:button and kin.

• afcf71c (Schema) A task hosts the same interactive forms as an
  exercise — the classifier always treated them alike.

• 6459c40 (Schema) choices/@multiple-correct, choices/@cols, a
  choice holding several blocks, and query/@visibility.

• 9cc3f82 (Schema) Codelens checkpoint prompts and feedback are
  marked-up text, not block sequences; program/@starting-step is the
  integer the trace extractor arithmetics on, not a yes/no; and
  program/@datafile joins its long attribute list.

• 00cfcdc (Schema) Parsons, both layouts: @database, blocks/@reuse
  and @numbered, block/@name and @depends (partial-order
  grading), the @ref/@xml:id block-reuse pair, inline content for
  horizontal blocks, exercise-level tests — and the model's
  preamble/postamble become the program-preamble and
  program-postamble the code and samples have always used, standing
  as their own named patterns so a future unification with a
  program's interior preamble/postamble stays a contained decision.

• b2c67fa (Schema) A match holds premises and a response in either
  order, and either side alone is a distractor.

• c3859d4 (Schema) Clickable areas in their three authored forms —
  paragraphs, cline code (with @language), and tabular cells —
  freely interleaved, trailing components in any order.

• 83e9cf8 (Schema) The select exercise, whole: @questions lists
  the variant questions, @grade says how the graded variant is
  chosen (random, A/B experiment, first-with-others-viewable, or the
  reader's pick), @experiment-name for the A/B case.

• 8c8e6bb (Schema) The legacy fill-in (setup/var/condition),
  fully modeled — the machinery is completely implemented, interactive
  and static — with the literate source stating plainly that this form
  will NEVER move to the stable schema; its only future is retirement.

• 45602a4 (Schema) Dynamic fill-in growth: a bare eval or
  constructor as a test's comparison shorthand (the assembly
  substitutes the full form), de-expression/@context, text-level
  feedback, solution/@include-automatic.

• 4628eb3 (Schema) Timed exams (@time-limit with the results,
  timer, feedback, pause toggles) and group work
  (@groupwork/@groupsize) on their hosting divisions.

• d7ce0f3 (Schema) datafile/@hide; a video may carry a title,
  which the Runestone manifest consumes.

• 8079c50 (Schema) The dual (dynamic-plus-static) exercise, modeled
  minimally and marked experimental, matching the sample's own alert.

• 5f6a6bd (Schema) Slate housekeeping: the no-namespace input and a
  redundant xhtml:button special case are gone; the wildcard's
  comment explains the namespace-as-foreign-marker principle and the
  declare-once idiom.

• a3b1555 (Sample book) Five stale prose references to
  docinfo/programs now read docinfo/defaults/programs, the
  location the code prefers, the schema models, and the book itself
  uses.

Nearly all modeling deliberately lives in the development overlay; the
base schema changes are confined to the program element, the video
title, and one tabular-cell extension. Under the stable schema the
book's count falls 322 to 289 — the Runestone frontier remains
development-schema territory by design.

Testing: every schema regeneration is idempotent; the Guide holds its
two-error baseline; a full before/after HTML build of the sample book
differs in exactly the pages the source repairs touch, each verified
intended (including the show-eval page, whose buttons now arrive as
plain HTML tags with no namespace residue); and the sample article's
slate output sheds its now-pointless xmlns attributes while
rendering identically.

Claude Fable 5, acting as a coding assistant for Rob Beezer