docs: add .NET coding standards and reference from phase docs

Establish project-wide rules for testing (xUnit 3 / Shouldly /
NSubstitute), logging (Microsoft.Extensions.Logging + Serilog +
LogContext), and general C# conventions. Referenced from CLAUDE.md
and phases 4-7.

docs/plans/phases/phase-5-mapping-verification.md

@@ -10,6 +10,7 @@ Confirm zero unmapped items, validate all N/A justifications, enforce naming con
 
 - Phase 4 complete: all items have .NET mappings or N/A status
 - Verify with: `dotnet run --project tools/NatsNet.PortTracker -- report summary --db porting.db`
+- Naming verification must check compliance with the [.NET Coding Standards](../../standards/dotnet-standards.md)
 
 ## Source and Target Locations