feat: add shareable compare demo link

[Tiyatrotist]

Adds a shareable profiler.html?demo=compare mode that loads the existing A/B sample drift datasets automatically. The single-profile controller now reserves demo=compare instead of also loading its own sample, while the compare button and URL path share one loader. Closes #742.

Validation:

• node --check assets/profiler-ui.js: passed
• node --check assets/profiler-compare.js: passed
• git diff --check: passed
• pytest tests/test_js_parity.py -q: 28 passed, 1 pre-existing failure in quoted-newline delimiter parity
• the same quoted-newline parity test fails on a clean upstream/main worktree, confirming it is unrelated to this PR.

[vercel]

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[github-actions]

@yakew7 @ahmdkaml - new PR touching a path you own, please review.