Pass 2: drop main-imported tests that reference removed hand-written types

The structural merge brought in test files from main that reference
hand-written quantity classes which vectors deleted in favour of generated
types. Remove them so the tree compiles; equivalent coverage against
generated types is tracked in issue #54.

Removed:
- AcousticImpedanceTests.cs (FromRayls / FromPascalSecondsPerMeter / etc.
  not emitted by QuantitiesGenerator; per #48 only the SI base-unit factory
  is generated).
- AcousticDirectionalityIndexTests.cs (DirectionalityIndex type was
  hand-written; not in dimensions.json).
- ReflectionCoefficientTests.cs (ReflectionCoefficient and SoundAbsorption
  were hand-written; not in dimensions.json).
- SoundSpeedTests.cs (FromFeetPerSecond not emitted; static Multiply method
  not generated).
- NuclearQuantitiesTests.cs (FromGrays / FromMicrograys / FromRads /
  FromEnergyAndMass / FromMicrosieverts / Energy.FromJoules — none of these
  factory names are emitted).
- PhysicalDimensionTests.cs (uses BootstrapUnits.Meter and the old
  PhysicalDimension(baseUnit:, length:, time:) constructor; both removed).
- PhysicalDimensionExtensionsTests.cs (uses 1.0.Meters() / 1.0.Kilograms()
  extension methods that don't exist in the generator output).
- UnitExtensionsAndExceptionTests.cs (uses Units.Meter as an enum-like value
  with IsBaseUnit / IsSI / IsMetric / IsImperial extensions; vectors emits
  Units as a namespace of per-unit structs).

Also add ktsu.RoundTripStringJsonConverter PackageReference to
Semantics.Strings.csproj so SemanticString.cs's
[JsonConverter(typeof(RoundTripStringJsonConverterFactory))] resolves.

Author: claude

Semantics.Strings/Semantics.Strings.csproj

@@ -7,6 +7,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Polyfill" PrivateAssets="all" />
+    <PackageReference Include="ktsu.RoundTripStringJsonConverter" />
     <PackageReference Include="System.Memory" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
   </ItemGroup>
   <ItemGroup>