Replace Pixely.Architecture with Pixely.Fitness and ship the peach architecture document

[botoddly]

Summary

• Remove Pixely.Architecture, Pixely.Architecture.Testing, their test projects and the three docs/architecture-*.md files.
• Add src/Pixely.Fitness, the Peach architecture fitness functions moved from Babka. It now uses project references, references Pixely.Core directly, and reads registrations through a new internal ServiceCollection.Descriptors (InternalsVisibleTo("Pixely.Fitness")) instead of private reflection.
• Drop rule 30 UpdatablesSitInTheirBand and the DirectionStart / PresentationStart options: UpdateOrder cannot be read without instantiating the updatable, so the rule is not checkable statically. Games passing those options need a matching update.
• Add docs/peach-architecture.md and ship it in the NuGet package under docs/. buildTransitive/Pixely.props exposes PixelyDocsDirectory so a consumer can locate the version it references; README and the document itself describe this.
• Solution, packaging project, package integration tests and AGENTS.md updated accordingly.

Verification

• dotnet build Pixely.slnx: 0 errors.
• dotnet pack packaging/Pixely/Pixely.Package.csproj: nupkg contains docs/peach-architecture.md and the updated buildTransitive/Pixely.props.
• Pixely.DependencyInjection.Tests: 171 passed.
• Package integration tests are [Explicit] and were not run locally.

The parallel build hang hit during this work is tracked in #501.

[github-actions]

✅ Development package Pixely 0.0.50 published successfully.

Workflow run (attempt 1)