feat(docset): publish the layered documentation pipeline #3

Closes #2

Builds a complete static documentation site from a project folder.

• Articles: CommonMark blocks and inlines, pipe tables, task and nested lists, footnotes, emphasis extras, the full HTML entity and Unicode emoji tables, sanitized raw HTML.
• Authoring: front matter, block and inline includes with cycle checks, code excerpts by region, line range, highlight and dedent, alerts with custom kinds, tabs with dependent conditions, row layouts, figures, video, math, Mermaid and PlantUML diagrams.
• Reference: Pudu API pages from source declarations (modules, records, unions, aliases, traits, implementations, functions, constants) with filters, layouts, overwrite files, and source links; HTTP API pages from OpenAPI 3 and Swagger 2; structured API page files.
• Navigation: YAML, JSON and Markdown tables of contents, nested and folder tables, order, breadcrumbs, previous and next links.
• References: uid registry, <xref:...>, @uid, xref: links, and cross-reference maps read and written in YAML and JSON.
• Site: an accessible default theme with light, dark, print, landing and chromeless layouts, template overrides, search, sitemap, SEO metadata, edit links and last-updated dates from git, a not-found page, a manifest, and model exports.
• Command line: build, metadata, serve, help and version, with incremental publishing and dry runs.

Validation

pudu check $(find src test tools examples -name '*.pudu')   # no diagnostics
pudu fmt --check src test tools examples                    # clean
pudu lint src test tools examples                           # clean
pudu test test                                              # 13/13 suites, 376 assertions
pudu run examples/BuildSite.pudu                            # builds examples/site

Mutation testing over the pure layer runs on release pull requests into main.