Feat: Refresh docs, package assets, and core runtime concurrency

[rian-be]

Summary

This PR refreshes the repository and package documentation, adds NuGet package icons, and closes the remaining core runtime concurrency gap.

Added

• dedicated package README for ModularityKit.Mutator
• refreshed governance package README with package-specific visuals
• organized asset structure under brand, core, and governance folders
• NuGet package icons for both core and governance packages
• core runtime execution gate that enforces MaxConcurrentMutations
• focused core concurrency tests covering shared-state serialization and bounded parallelism

Changed

• root README is now a short repository overview instead of an outdated package document
• core and governance package READMEs now describe their actual package scope independently
• core runtime concurrency is now enforced instead of remaining a passive option
• same-state direct mutation execution is now serialized explicitly in the core runtime
• batch execution documentation now states ordered and deterministic concurrency behavior
• XML/API docs for core runtime concurrency now distinguish core execution from governance request concurrency

Result

The repository documentation is aligned with the current package structure, NuGet packages now have explicit icons and package READMEs, and core mutation execution concurrency is now deterministic, documented, and covered by tests.

Testing

• not run locally

Linked Issues

• Closes [Task]: Enforce core runtime concurrency semantics #18

Checklist

• Root README reflects the current repository structure
• Core package has a dedicated package README
• Governance package has a dedicated package README
• NuGet packages include package icons
• MaxConcurrentMutations affects real core runtime behavior
• Shared-state concurrency semantics are explicit and test-covered
• Batch concurrency behavior is documented and deterministic