Prune /prototyping to only what we intend to ship

Index now lists eleven entries — three design-review pages, three
banner-grammar prototypes, and six journey demos — instead of twenty.
The rolled-back inline-* and per-example demos demonstrated layout
strategies we explicitly decided against; keeping them in the index
sent mixed signals about what the recommended grammar is.

Removed prototypes:
  example-mutability.html      inline-between-as-canonical (rolled back)
  example-closures.html        same
  example-for-loops.html       same
  example-slices.html          same
  layout-above.html            single-col-stacking variant (rolled back)
  layout-after-output.html     same
  layout-banner-top.html       2-col + figure-banner-row (only 1 figure)
  layout-banner-bottom.html    same
  layout-prose-aside.html      float-inside-prose (had bugs)

Builder cleanup in scripts/build_prototypes.py:
  - dropped build_example() (only used for the rolled-back demos)
  - dropped figure() helper (only banner() is needed now)
  - dropped VARIANT_CSS dict (variant CSS rules no longer relevant)
  - simplified render_cell() to a single 2-column form (figures live
    in banners, never inside cells)
  - simplified render_article() to take only banners=

Production rendering in src/app.py and src/marginalia.py is unchanged
in this commit; production migration to the banner grammar is still
pending review.

https://claude.ai/code/session_01MazwoRWAihW6dwso3fMCHE

Author: claude