Extract parseCommitHref, isSimpleTypeSelector, subtractIdMap, and
isCommitHash into a new viewer-util.js ES module so they can be tested
without a browser. Convert ifcurl.js from an IIFE to an ES module
importing from viewer-util.js; update footer.tmpl to type="module".
Add node:test suites for viewer-url.js (32 tests) and viewer-util.js
(28 tests). Add npm test script and document it in the build section.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>