FastMoq v4: introduce provider-first architecture, package split, and migration docs

[cwinland]

Summary

This PR captures the FastMoq v4 release line relative to tag 3.0.0.

Compared to 3.0.0, FastMoq now moves from Moq-centric internals to a provider-first architecture, introduces a clearer package split, adds provider-neutral verification and scenario-building APIs, and expands the migration and release documentation needed to adopt the new model.

What changed

• introduced provider-first abstractions and registry support through IMockingProvider, IMockingProviderCapabilities, IFastMock<T>, and MockingProviderRegistry
• added the built-in reflection provider plus provider packages for Moq and NSubstitute
• split the package layout so provider contracts live in FastMoq.Abstractions, EF/DbContext helpers live in FastMoq.Database, and provider-specific adapters live in FastMoq.Provider.*
• moved DbContext-specific implementation details out of core while preserving the main FastMoq namespace call shape for end users
• added provider-neutral verification and logging helpers, including Verify(...), VerifyLogged(...), and TimesSpec
• added fluent scenario-building support with Scenario.With(...).When(...).Then(...).Verify(...)
• replaced older coupled option flows with more explicit creation, invocation, and policy surfaces such as InstanceCreationFlags, InvocationOptions, BuiltInTypeResolutionFlags, and known-type registration support
• narrowed compatibility behavior around Strict, kept MockOptional as an obsolete bridge, and moved Moq-specific HTTP compatibility helpers into FastMoq.Provider.Moq
• expanded test coverage, executable examples, generated API documentation, migration guidance, and release notes for the v4 line

Breaking / migration notes

• Strict is no longer the old all-in-one strictness switch; broader preset behavior should use the explicit preset APIs
• strict IFileSystem behavior now stays enriched through FastMoq's built-in known-type pipeline instead of behaving like a raw empty Moq mock
• older Moq-oriented HTTP setup helpers remain available for migration, but now come from FastMoq.Provider.Moq
• existing suites can stay on the compatibility path, while new or actively refactored tests can move toward GetOrCreateMock(...), provider-neutral verification, and explicit provider selection

Documentation

• refreshed the post-3.0.0 release summary
• aligned the repo and docs landing pages with the new package and release framing
• kept the migration and breaking-change guidance pointed at the same release story

Notes

This is a major release delta from 3.0.0, not a small compatibility patch. The focus is to preserve a practical migration path for existing Moq-heavy suites while establishing the provider-neutral architecture and package boundaries for v4 and beyond.

[cwinland]

Fixes #39
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