Add read-only DDD repository contracts and EF base types

[MarkDerman]

Summary

• Introduce IReadOnlyRepository<TAggregateRoot> and make IRepository<TAggregateRoot> extend it, so read-only and read-write repository contracts are explicit.
• Split the EF base implementation into a read-only base with no-tracking query defaults and a write base layered on top.
• Add a read-only identity repository base and contract tests covering the new inheritance shape.

Testing

• dotnet test DomainDrivenDesign/Tests/Tests.Odin.DDD.csproj -f net10.0 --filter "Category!=IntegrationTest"
• dotnet test DomainDrivenDesign/Tests/Tests.Odin.DDD.csproj -f net9.0 --filter "Category!=IntegrationTest"
• dotnet test DomainDrivenDesign/Tests/Tests.Odin.DDD.csproj -f net8.0 --filter "Category!=IntegrationTest"
• Validation passed; existing repo warnings about the missing System/Strings project reference and known package advisories remain.