[6.x] Debounce LinkFieldtype URL writes

[jasonvarga]

Summary

Reduces churn in the Link fieldtype with two small changes scoped to resources/js/components/fieldtypes/LinkFieldtype.vue:

• Wraps the URL-sync logic in a 150ms debounce (created in created() and markRaw-wrapped so Vue 3 doesn't proxy the function and break the debounce internals). Typing in the URL field no longer fires update + updateMeta on every keystroke.
• Adds an identity short-circuit (meta === oldMeta) ahead of the existing JSON.stringify check in the meta watcher so unchanged meta references skip the deep comparison entirely.

This is one slice of the broader CP performance improvements from #14512