feat(core): reorganize Core source layout

[DennisDyallo]

Summary

• reorganize Core into Abstractions, Devices, Sessions, Transports, Protocols, Native, and Utilities slices
• update Core test layout, namespaces/usings, examples, consuming modules, and active docs

Verification

• dotnet toolchain.cs build
• dotnet toolchain.cs -- test --project Core --smoke
• dotnet toolchain.cs -- test --smoke
• dotnet toolchain.cs docs-qa
• dotnet format whitespace --no-restore --verify-no-changes
• dotnet format style --no-restore --verify-no-changes

Note: full dotnet format --no-restore --verify-no-changes still exits nonzero on existing IL2026/IL3050 analyzer warnings in src/Tests.TestProject/Program.cs.