feat(gp-sphinx[fonts]): default preload covers heading/italic/bold-code FOUT

[tony]

Summary

• Expand DEFAULT_SPHINX_FONT_PRELOAD from the existing trio (Sans 400, Sans 700, Mono 400) to cover every face Furo / furo-tw.css is observed to demand above the fold.
• Eliminate first-paint FOUT on bold headings, italic body text, and bold inline code by default — downstream consumers no longer need to extend the list locally.

Why these specific faces

Face added	Surface that needs it
Sans 500 normal	h1–h6, sidebar labels
Sans 600 normal	blockquote.epigraph
Sans 400 italic	<em> body text, announcement-bar emphasis
Mono 700 normal	bold inline code (<strong><code>, MyST **\code`**`)

These were discovered while debugging first-paint FOUT on libtmux-mcp (tmux-python/libtmux-mcp#36). All four are pulled by Furo's own CSS, not project-specific code, so every gp-furo consumer that renders any of these surfaces ships the same FOUT until they extend the preload list themselves. Moving the additions upstream means new sites get correct first-paint behaviour for free.

Changes

• packages/gp-sphinx/src/gp_sphinx/defaults.py: Append the four tuples; reorder so faces are grouped by family with weights ascending. Doctest len(DEFAULT_SPHINX_FONT_PRELOAD) bumped from 3 to 7. Docstring expanded to name each face's purpose so the constant explains itself.
• tests/test_config.py: test_merge_sphinx_config_default_fonts length assertion bumped from 3 to 7 (matches the existing brittle-by-count style of nearby asserts).
• docs/configuration.md: Font defaults table cell for the constant shortened to a one-line summary so it stays readable as the list grows.
• CHANGES: New ### What's new entry under ## gp-sphinx 0.0.1 (unreleased) describing the FOUT impact.

What is NOT in scope

• Adding more faces (Sans 300, Mono 500/600, more italics). The constant is intentionally minimal — every preload byte contends with critical CSS/HTML on the network. Only faces with documented above-the-fold use go in.
• Promoting the constant from gp-sphinx defaults into sphinx-fonts itself. The current ownership (gp-sphinx ships the IBM-Plex-flavoured defaults; sphinx-fonts is theme-agnostic and registers the config option) is correct.
• Editing libtmux-mcp's local override. That cleanup happens in libtmux-mcp once a gp-sphinx release ships these defaults.

Test plan

• uv run ruff format . — formatting unchanged
• uv run ruff check . --fix --show-fixes — lint clean
• uv run mypy — types clean across 221 source files
• uv run pytest --doctest-modules packages/gp-sphinx/src/gp_sphinx/defaults.py — doctest passes with new length
• uv run pytest tests/test_config.py::test_merge_sphinx_config_default_fonts — assertion passes with == 7
• uv run pytest — full suite green (1473 passed, 161 skipped)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.57%. Comparing base (6736f2a) to head (d3a40f3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #34   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files         205      205           
  Lines       16814    16814           
=======================================
  Hits        15398    15398           
  Misses       1416     1416           

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