Delegate unmatched application requests

[samuel-williams-shopify]

Summary

• register the default application index as an explicit GET / route
• pass unmatched requests to the middleware delegate
• document how applications can opt into SPA-style history fallback

This makes Lively::Application compose normally with other HTTP middleware. Previously, every unmatched path rendered the application index and the configured delegate was never consulted.

Compatibility

Applications relying on unknown paths rendering the index can preserve that behavior by overriding #handle.

Verification

• bundle exec sus
• bundle exec rubocop