Alternative: render from_markdown via markdown-it-py

[MattFisher]

What this is

An alternative implementation of Post.from_markdown() that delegates parsing to markdown-it-py (a CommonMark parser) plus the standard footnote plugin, with a small renderer (mdrender.py) that maps the syntax tree onto Substack's node schema. Node construction is centralised in a new nodes.py module so the (undocumented) schema lives in one place.

Why I'm opening it

First off — this approach wasn't discussed beforehand, and I realise it's a bigger change than a typical PR, so I'm putting it up purely for your consideration; entirely your call on whether it's a direction you want to take.

I started out adding footnote support to the existing hand-rolled parser (#56). As that grew — footnotes, fenced/inline-code edge cases, multi-paragraph definitions — it started to feel like we were re-implementing a Markdown parser. So I prototyped this alternative to see how it compared, and it turned out significantly simpler:

• from_markdown() drops from a ~270-line hand-rolled parser to a few lines delegating to the renderer (net post.py ≈ −215 lines).
• Footnotes come essentially for free from the footnote plugin (including multi-paragraph definitions), rather than via bespoke pre-parse text extraction.
• A real CommonMark parser brings correctness for free (nested structures, edge cases) and removes the fragile overlapping-regex inline parsing.

Trade-offs (flagging honestly)

• Adds two runtime dependencies: markdown-it-py and mdit-py-plugins (both widely used and well maintained). This is the main thing to weigh.
• Two intentional, CommonMark-correct behaviour changes vs the old parser (tests updated to match, with comments):
  • Consecutive > lines are one paragraph; blank > lines split paragraphs (standard CommonMark). The old parser made one paragraph per line.
  • Footnote definitions that are never referenced are dropped (not appended to the end).
• parse_inline() / tokens_to_text_nodes() remain as public helpers (still used by the manual footnote() builder), but from_markdown() no longer relies on them.

Tests

• All existing from_markdown/parse_inline tests pass (the two semantic-difference tests above were updated).
• Added test_from_markdown_features.py with end-to-end coverage of every feature listed in the from_markdown() docstring (headings 1–6, bold/italic/bold+italic/inline-code/strikethrough, links, images, linked images, code blocks with/without language, blockquotes, bullet/ordered lists, horizontal rules, paragraphs).
• Footnote tests include references/definitions, named labels, numbering, multi-paragraph definitions, and code-span safety. 86 passing (excludes the pre-existing live-API tests that require credentials).

Relationship to #56

This is an alternative to #56 — if you'd prefer this approach, #56 can be closed in its favour; if not, no harm done and #56 stands on its own.