Expose render_html and make the PDF stack an optional extra #2

Problem

Scriptorium can only produce a PDF. For documents the pipeline is already pure HTML + CSS: the preprocessors (footnotes, citations, glossary, timeline) → parse() → emit() build one HTML document, and WeasyPrint only prints it. But that HTML exists only as a local inside render_pdf (scriptorium/galley.py:668), and galley.py imports weasyprint at module level (galley.py:19). So:

1. No HTML output. Nothing returns the HTML a document renders to, so a screen consumer has to re-implement the preprocessor chain to get it. The chain's order is deliberate (citations after footnotes, glossary last), so a copy drifts.
2. WeasyPrint is mandatory. It pulls in pango and friends. A consumer that only needs Markdown → HTML still has to install the whole PDF stack.
3. The entry point lives in galley.py, which is being retired: only the deck path still uses measure() / pack().

First consumer

marginalia (apiad/marginalia#5) is being rebuilt on scriptorium so its reader inherits all of scriptorium's Markdown features (components, math, footnotes, citations, glossary, cross-refs). It needs the HTML of a document and never a PDF. It will run on a server where installing WeasyPrint is pure cost.

Checked on 2026-10-04: a raw <span class="marg-s" data-o="42"></span> block survives process_footnotes → parse → emit as its own <div class="unit">, in order, between a footnoted paragraph, a loose list and the endnotes. HTML comments are already stripped by _split_md, so marginalia's annotations (HTML comments in the chapter source) never reach the PDF.

Wanted

• render_html(src, *, theme_name=None, cwd=None, execute=True, vars=None, code_root=None, project_meta=None) -> (html, warnings) in a module outside galley.py, holding everything render_pdf does up to and including emit().
• render_pdf becomes render_html + WeasyPrint, with unchanged behaviour and report.
• WeasyPrint moves to an optional extra (scriptorium[pdf]) and is imported lazily inside the PDF path. import scriptorium and render_html work without it; render_pdf without it fails with a message naming the extra.
• scriptorium render --html (or -o x.html) writes the HTML document. A deck theme still needs measuring and the pdf extra, so --html on a deck errors out clearly.
• Tests: render_html round trip on the existing fixtures, render_pdf output unchanged, and an import test in an environment without WeasyPrint.

Note: parse.py and tests/test_parse.py carry uncommitted changes in the shared checkout on zion; this work should not need to touch either.