Add optional `role` field to `CustomElementDeclaration`

`ElementInternals` allows element authors to assign implicit roles to elements. `CustomElementDeclaration` should have an optional `role` or `implicitRole` field to document this. Tooling like aXe, lighthouse, or `cem mcp` should use this to perform audits.

Start at the CustomElementDeclaration definition and inspect how declaration fields are represented and validated. Check the existing manifest schema and tests before resolving whether the optional field should be named role or implicitRole; done means aXe, Lighthouse, or cem mcp can consume the documented role information.

Written by the indexing model from the issue text.