V5 tracker: Move bundled provider defaults out of FastMoq.Core and tighten package composition

[cwinland]

Docs reference: docs/roadmap/README.md

This tracker plans the v5 package-composition cleanup that keeps FastMoq.Core provider-neutral and moves bundled provider defaults to the aggregate package.

Acceptance checklist

• The intended responsibilities of FastMoq.Core, the aggregate FastMoq package, and explicit provider packages are explicit.
• The default provider-selection and registration story is explicit for core-only, aggregate, and explicit-provider installs.
• The migration path for consumers relying on current bundled defaults is documented clearly enough for implementation planning.
• Follow-on implementation can proceed without leaving package-default behavior ambiguous.
• Related provider-expansion work such as #128 can depend on this package-composition direction cleanly.

Follow-on boundaries

• this issue plans package composition; it does not itself implement provider expansion
• generator package rules stay in #120 and #127