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Part 1 of 3 for #13. Spec: docs/superpowers/specs/2026-10-20-html-book-design.html; plan: docs/superpowers/plans/2026-10-20-html-book.html (tasks 1-3).

What changed

• project.py: outputs: in scriptorium.yaml becomes Project.outputs: list[Output]. Each output has a name and a format (pdf or html); a pdf takes file, page, vars, an html output takes dir, vars, cover, image-max-width, js, css. An unknown key, a key of the other format, or an unknown format raises ProjectError naming the output and the key. No outputs: means one pdf output writing book.pdf, so existing projects are unchanged.
• prepare(..., page=) and render_pdf(..., page=) merge a page block (size, margin) over the theme's for one render. A5 joins the named sizes.
• scriptorium render X.yaml builds every output and prints one line each; --only NAME (repeatable) picks; -o is refused when more than one output is selected. --format {pdf,html} is for single files. HTML outputs exit 2 with "not available yet" until part 2.
• README "Several outputs" section, CHANGELOG entry, and the spec and plan.

Measured

• tests/test_outputs.py: 14 new tests, all seen failing first. Locally test_outputs, test_book, test_furniture, test_galley pass; CI runs the rest.
• Real CLI on a two-output project: pdfinfo reports 419.528 x 595.276 pts for the A5 output and A4 for the other.

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