docs(site): mobile-first revamp of all pages

Rewrite both stylesheets mobile-first (base = phone; @media min-width
progressively enhances) and add a responsive nav.

- Hamburger menu on small screens: links collapse into a clean, uniform
  dropdown list (plain rows + separators, no out-of-place boxed buttons);
  full inline nav with boxed CTA returns at >=720px. Auto-wired for every page.
- Fix horizontal-overflow root causes: min-width:0 on grid/flex code containers,
  pre/sig scroll internally, long names wrap (overflow-wrap). 0 page overflow on
  every page type at 375px.
- Stack layouts on mobile: single-column hero/features/steps/index grid,
  column parameter cards, 2-col color swatches; multi-column returns on wider
  screens. Reduced hero/section padding; fluid clamp() headings.
- Hover cards gated to hover-capable devices so a tap just navigates.
- Desktop layout preserved (verified at 1280px: inline nav, 2-col hero, 3-col
  features, no regression).

Verified in-browser at 375px and 1280px across landing, API index, item
(method/keyword/annotation/phase), namespace overview, and color pages.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/gen.py

@@ -188,6 +188,7 @@ def head_html(title, desc, css_file):
 def topbar(cfg):
     return ('<header class="nav"><div class="wrap nav-in">'
             '<a class="brand" href="index.html"><span class="logo">L</span> %s</a>'
+            '<button class="nav-toggle" aria-label="Toggle menu" aria-expanded="false">☰</button>'
             '<nav class="nav-links">%s</nav></div></header>') % (esc(cfg["brand"]), nav_html(cfg))
 
 # ---------------------------------------------------------------------------