Support opt-in template transclusion on Markdown pages

[JeroenDeDauw]

For #4

Add $wgNativeMarkdownTemplateTransclusion (default false). When on, {{...}}
on Markdown pages expands via MediaWiki's parser, so infobox, citation and
navbox templates render with dependency tracking (editing a template reparses
the pages using it), recursion/size limits, and wikitext-level sanitization.
Own-line calls render as block HTML (infoboxes are not paragraph-wrapped and
may span multiple lines); inline calls render within text. The pure Application
layer stays MediaWiki-free behind a TemplateExpander port; the adapter reuses
one parser per render and merges each child ParserOutput into the page.

Delegating to the parser also enables parser functions and magic words inside
the braces, amending the SPEC's "no parser functions or magic words" non-goal
(approved when directing this build). Deferred, and documented in the README:
<ref>/<references/> in the Markdown body and cross-call citation state,
{{:MarkdownPage}} full-page transclusion, and subst.

Built at @JeroenDeDauw's direction: design-first, with two independent Fable design reviews before implementation, and amending the SPEC's no-parser-functions decision per his go-ahead. Context: the NativeMarkdown codebase and the MediaWiki 1.43 parser APIs verified against core.
_{Written by Claude Code, Opus 4.8 (max)}