Premium Analytics: update Stats widget porting guidelines

[dognose24]

Fixes #

Proposed changes

• Update the Premium Analytics Stats widget porting guidelines based on recent widget ports.
• Document expectations for full-bleed presentation, canonical Storybook stories, dashboard comparison coverage, state wrappers, unsupported error copy, drill-down behavior, header controls, default dashboard registration, and data-layer prep PR scope.
• Add a Premium Analytics changelog entry.

Related product discussion/links

• None.

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• Review projects/packages/premium-analytics/AGENTS.md and confirm the guidance matches the current Stats widget porting conventions.
• Confirm the changelog entry is present.

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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