docs(style-guide): update typography + hero doctrine to MonoLisa-only

Align the style guide with the new 1-font system (§2.1.4, §5.1, §5.2),
rewrite the hero construction (§5.3), update section-header and masthead
specs (§6.3, §6.4), fix the hero load sequence (§8.1), replace the CSS
skeleton's Google Fonts link with GCS MonoLisa preloads (§11), and update
brand-application notes (§12). Flag §7.1 with a status note since the
terminal box now scopes to the plot-of-the-day panel only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MarkusNeusinger