V5 tracker: Remove obsolete compatibility surfaces and narrow legacy Moq-shaped APIs

[cwinland]

Docs reference: docs/roadmap/README.md

This tracker narrows or removes obsolete compatibility surfaces so provider-first APIs become the default public guidance in v5.

Acceptance checklist

• The targeted obsolete or compatibility-only public surfaces are enumerated explicitly rather than described generally.
• The provider-first replacement guidance is explicit for each removed, narrowed, or newly-obsolete legacy API.
• Migration-documentation and analyzer dependencies are identified before removal begins.
• The cleanup plan distinguishes between APIs removed outright, APIs newly marked obsolete, and APIs deferred beyond v5.
• Follow-on implementation can proceed without leaving legacy-surface behavior or migration guidance ambiguous.

Follow-on boundaries

• optional-parameter compatibility cleanup stays in #142
• generator helper-surface work stays outside this issue