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-4-dotnet-design.md

@@ -10,6 +10,7 @@ Every module, feature, and test in the porting database must have either a .NET
 
 - Phases 1-3 complete: all Go items in the DB, all libraries mapped
 - Verify with: `dotnet run --project tools/NatsNet.PortTracker -- report summary --db porting.db`
+- Read and follow the [.NET Coding Standards](../../standards/dotnet-standards.md) — all naming, testing, and logging decisions must comply
 
 ## Source and Target Locations
 
@@ -149,7 +150,7 @@ dotnet run --project tools/NatsNet.PortTracker -- test map <id> \
   --db porting.db
 ```
 
-Go test naming (`TestParserValid`) translates to .NET naming (`TryParse_ValidInput_ReturnsTrue`). Each Go `Test*` function maps to one or more `[Fact]` or `[Theory]` methods. Table-driven Go tests often become `[Theory]` with `[InlineData]` or `[MemberData]`.
+Go test naming (`TestParserValid`) translates to .NET naming (`TryParse_ValidInput_ReturnsTrue`). Each Go `Test*` function maps to one or more `[Fact]` or `[Theory]` methods. Table-driven Go tests often become `[Theory]` with `[InlineData]` or `[MemberData]`. Use Shouldly for assertions and NSubstitute for mocking — see the [.NET Coding Standards](../../standards/dotnet-standards.md) for details.
 
 ### Step 4: Mark N/A items