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Fixes #8.

What changed

• galley._lang() reads lang: from the frontmatter or scriptorium.yaml first, then from the theme's top-level lang:, and falls back to en. emit() and emit_deck() write the result into <html lang>, and WeasyPrint uses that attribute to pick its pyphen hyphenation dictionary. The value lands inside an HTML attribute, so anything that is not a BCP 47-shaped tag raises ValueError.
• project.py carries lang through as a content key.
• uh defaults to lang: es. Its description is already Spanish, and it exists for Universidad de La Habana documents. No other theme changes.
• Docs: README, the authoring know-how, the CHANGELOG.

Measured

• tests/test_lang.py has 5 new tests. They render 30 copies of "desarrollo" in a 12 mm column, which is narrower than the word, so the column has to hyphenate:
  • With the default, the English break desar‐ appears.
  • With lang: es, every break is one of de‐, desa‐, desarro‐, and desar‐ never appears.
  • The uh theme with no lang: key behaves like Spanish.
  • A lang in scriptorium.yaml reaches the render.
  • A value carrying a quote raises ValueError.
• The first version of the fixture used a 19 mm column, and every test passed whatever lang was set, because each word fit on its own line. That is why the column is now 12 mm. With the lang entry removed from the project allowlist, the project test fails.
• Full suite: 267 passed locally.

Note

This PR branches from main before #10, the CI workflow, so it gets no check run until #10 merges and this branch is rebased onto it.

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