docs(theme-neutral): document Figtree and its loading snippet - #6079

The README and the CLI theme docs described Neutral as a system-font
theme while neutralTheme declares Figtree for body and heading. Figtree
is not bundled or loaded, so a clean consumer app silently falls back
to system fonts and the missing webfont reads as an integration
failure.
Describe Neutral as Figtree typography in both places, add the
Stone-style Fonts section with the Google Fonts snippet to the README,
and guard README/theme drift with a colocated test that derives the
declared webfonts from the resolved --font-family-* tokens.
Fixes facebook#5991 

Some apps picked Neutral precisely because nothing external loads
(offline apps, no-third-party-request policies). Frame skipping the
webfont as a supported choice rather than a failure state, so the new
Fonts section keeps that audience.
Part of facebook#5991 

Widen the README guard to the theme's whole font surface: every
--font-family-* token by prefix plus component-level fontFamily
overrides, with var() aliases (nested included) stripped before
parsing. Tie the loading-recipe assertion to each declared webfont
(family= URL token or @font-face) instead of a blanket regex the
mutation run proved vacuous. Add a CLI-side guard pinning the
astryx docs theme Neutral row via collectUnloadedFonts, so the
system-fonts claim cannot return on the surface the README test
cannot see.
All three guards are mutation-proven: URL font swap, undocumented
webfont injection, and doc-row revert each fail exactly the
intended assertion.
Part of facebook#5991