docs(templates): design func template constraints - #5670

Resolves #5643.

Adds the template-engine-constraints design. The other templating designs leave workload and extension bundle checks to it. Docs only, and it builds on the reconciled designs in #5663.

• Templates declare requirements in the standard TemplateEngine constraints section. Func adds func-workload, which checks the workloads the CLI loaded, and func-bundle, which checks the project's resolved extension bundle. Both take NuGet version ranges.
• Only func template packages can use the func types. dotnet new blocks templates with constraint types it doesn't know, so packages shared with it can't use them.
• The engine's os and host checks stay. The .NET SDK sdk-version and workload checks aren't registered, so templates that use them are blocked.
• Results map to the four eligibility states in the integration design. Anything that isn't eligible is blocked, even with --force.
• The CLI writes every next step itself, and the only template text it uses is a validated workload name.
• Azure-Samples templates declare one workload constraint per stack. Choosing templates by target framework stays out of constraints.

The coordination inventory now lists the constraints design as complete, and the integration, quickstarts, and Samples designs point to it.