Add an algorithm visualizer

Write a page into algo_demo/ that behaves like the other 36.

Invocation: /lc-algo-demo <algorithm> — e.g. /lc-algo-demo trie.

Why the contract matters here specifically

algo_demo/ is 36 hand-written pages plus common.js and style.css, with no generator. site/build.sh copies the directory wholesale. So nothing compiles these pages, nothing templates them, and every shared behaviour is held together by convention alone — which means a page that reimplements one of them looks fine in isolation and is wrong the moment the theme changes, the window resizes, or someone repaints the palette.

Anything that should look or behave the same on all 37 pages belongs in common.js or style.css, never copy-pasted into a page's inline <script>.

site/test/algo-demo.test.js evaluates the shipped common.js in jsdom and asserts against it, so a change to the shared files is testable. A change copy-pasted into a page is not.