docs: clarify declarative fixture testing guidance

[janechu]

Pull Request

📖 Description

Clarifies agent-facing testing guidance for declarative template work:

• Explains that declarative syntax, template generation, hydration behavior, fixture category behavior, and other user-authored template behavior should be covered with browser-backed declarative fixtures.
• Points agents to the local fixture README files so they can choose the right fixture category and follow category-specific examples.
• Reinforces in the fixture writing guide that parser-only tests should supplement fixture coverage rather than replace it.

👩‍💻 Reviewer Notes

This follows review feedback on #7638, where declarative syntax coverage should use the existing browser fixture suite instead of only source-level parser tests.

📑 Test Plan

• git diff --check
• npm run checkchange

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Agents

• I have linked to an existing issue in this project that this change addresses
• I have read the skills
• I have read the DESIGN.md file(s) in packages relevent to my changes
• I have updated the DESIGN.md file(s) in packages relevent to my changes