fix: keep first duplicate declarative template assignment

[janechu]

Pull Request

📖 Description

• Allows duplicate connected <f-template> publishers with the same name to keep the first resolved declarative template assignment instead of rejecting the definition.
• Adds source-level and declarative fixture coverage for duplicate template names with a simple bound element.
• Updates fast-element design/README docs, fixture docs, and generated size documentation.

👩‍💻 Reviewer Notes

Please focus review on the first-publisher semantics for duplicate <f-template> names and the new declarative fixture coverage.

📑 Test Plan

• npm run checkchange -- --branch origin/releases/fast-element-v3 --no-fetch
• npm run build (validation agent reported passed)
• npm run biome:check (validation agent reported passed)
• Full declarative Playwright suite, 531 tests (validation agent reported passed)
• Focused duplicate template tests (validation agent reported passed)

✅ 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.