Frontmatter number_headings selects the heading numbering; refs='ids' numbers on request

[PiotrCzapla]

Stacked on #59 (the h1 title boundary); the base flips to main once that merges.

What changes

• A document's frontmatter number_headings: legal (or decimal) selects the heading numbering whenever a call gives no scheme. mdhtml2html and mdhtml2typst read it from the Mdhtml result's meta; md2gfm extracts it from the raw source by the dialect's own frontmatter rule (new native frontmatter_meta). The call argument still wins when both are present. A plain string source carries no meta, so nothing changes for callers passing raw MDHTML.
• refs='ids' (the live-preview mode Solveit renders in) used to skip numbering entirely. A requested scheme now numbers the fragment's headings there too; nothing numbers automatically without a registry, as before.
• viewmd on a notebook reads the frontmatter message through fastcore's nb_frontmatter, since dlg2md fences that raw cell as code and the parser never sees it. viewmd rspa.ipynb therefore numbers the whole contract with the scheme the dialog declares.
• README and DIALECT.md document the key; md2html/viewmd help text says the default is the frontmatter's scheme.

Tests

test_frontmatter_selects_numbering covers the HTML fallback, argument precedence, the bare-string case, an unknown scheme name raising, ids mode numbering on request and never automatically, GFM, and Typst. The viewmd test gains a notebook whose raw frontmatter cell names the scheme. Full suite: 930 passed (the one failure, test_math_callbacks_with_math_core, is the pre-existing missing optional dependency). cargo test, cargo fmt, and chkstyle clean apart from the pre-existing note in chunk.py.

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