demo: side-by-side view, large-input bench mode, shareable URL state

[krkarma777]

Ideas for the hosted demo (https://krkarma777.github.io/string-diff/):

• Side-by-side pane rendering (current view is unified inline).
• A "benchmark me" button that generates a large input pair locally and reports timing — shows off the ms-level speed without users hunting for big texts.
• Encode inputs + options in the URL hash so diffs are shareable.
• Show entry/range counts alongside the timing readout.