feat(samples): add visual layout demo

samples/layout-demo/layout-demo.cs renders six representative layouts
(grow ratios, wrap+gap, justify/align, nesting, absolute insets, RTL)
two ways: ASCII boxes in the terminal, and with --html a page that
draws the engine's computed coordinates beside the browser's native
CSS flexbox for identical styles - a direct visual correctness check.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: StevenTCramer