[ledsun]

Allow users to run a custom Ruby WASM binary containing additional gems with the existing browser script loader:

<script
  src="./browser.script.iife.js"
  data-ruby-wasm-url="./ruby-with-custom-gems.wasm"
></script>

For WASI Preview 1, data-ruby-wasm-url replaces the default ruby+stdlib.wasm URL. Relative URLs resolve against the HTML page URL. Omitting the attribute preserves the existing CDN behavior. Empty values and surrounding whitespace are rejected, and load or compilation failures do not fall back to the CDN.

The attribute parsing is isolated in parseDataRubyWasmUrl(). The README includes a custom-gem example, and ten individual browser tests cover URL selection, Ruby startup, invalid values, and failures.