help-model epic: the document model + design note (slice A)

[helly25]

What (help-model epic #154, slice A)

The foundation for collapsing the two parallel help systems - the imperative
WriteReference walk and the 16 hand-assembled Render* string-builders +
kHelpText - into one semantic document model that pluggable backends render.
This slice adds the model + the design record; no live rendering is rewired yet.

Design (docs/design-help-model.md)

• Model is an output-independent AST; backends are plain text / ANSI color /
  Markdown / roff man page / HTML.
• Width is a text-backend-only concern (--width=N > detected TTY width via
  TIOCGWINSZ/$COLUMNS > 80 fallback); Prose free-flows, Rows/Entry wrap with a
  hanging indent, Example stays verbatim; Markdown/HTML/roff never hard-wrap.
• Highlighting is typed inline runs; cross-references are semantic
  RefTargets (Topic/Flag/Primary/ManPage/Url/Anchor), never formatted links;
  every Section/Subsection/Entry has a stable anchor, a BuildIndex walk
  yields the index, and a resolution pass validates every internal ref at build
  time (a dead --help=X becomes a build error).
• Authoring is C++ model data. The only string markup is backticks (single
  -> inline Code, triple fence -> Example); we deliberately do NOT parse
  #/-/* because those are exactly xff's flag/glob/regex characters. XHTML /
  Markdown are output backends only, never input.

Model (xff/cli/help_model.h)

Typed Inline runs (Text/Code/Emphasis/Strong/Ref + RefTarget); a properly
recursive block Content (Entry.details and Subsection.children nest, via a
std::vector indirection that also bounds the variant size); Rows / Bullets /
Example (with a lang hint) / SeeAlso; anchors on Section/Subsection/Entry;
Document root.

Tests

help_model_test.cc builds a document with every node kind and two levels of
Subsection nesting, then walks it with std::visit - proving the recursion
compiles and terminates - and asserts refs carry a semantic target and
entries/examples carry their metadata.

Next slices: the backtick parser, Render(Document, Backend) + porting the roff/
markdown/plain backends (byte-identical), build-from-SSOT, then the topic
migration that retires the Render* string pile.