C# generator drops the default on Quantity members, so e.g. gravity starts at 0 in C# but -9.81 in C++ #286

What's wrong

The C# and C++ generators produce different starting values for a Quantity member that has a number default:

• C#: CSharpCodeGenerator.DefaultLiteralFor (Schema/Generation/CSharpCodeGenerator.cs, around lines 805-829) has no arm for Quantity members. InitialiserFor therefore returns nothing, and the generated property keeps its type's zero value.
• C++: the C++ generator does initialise these members, via Schema.Cpp/CppFileBuilder.cs around lines 291-293 and QuantityInitialiser around lines 327-345.

Reproduction

A member Body.Weight : Quantity(Mass) with default 2.5 passes validation with no issues. The generated C# is:

[SchemaDefault(2.5D)] public ktsu.Semantics.Quantities.Mass<float> Weight { get; set; }

There is no initialiser, so Weight starts at 0. The C++ output starts it at 2.5.

The repository's own sample hits this too: samples/modernised.schema.json (around lines 529-540) declares gravity : Acceleration1D with default -9.81. C# objects start with zero gravity, while C++ objects start at -9.81. Data that leaves the field unset therefore behaves differently depending on the language.

PR #227 fixed #218 for Semantic and Vector members and noted in its body: "Out of scope: a NumberDefault on a Quantity member still gets no C# initialiser." No issue has tracked that remainder until now.

Suggested fix / acceptance criteria

• Magnitude and 1D quantities: emit = ktsu.Semantics.Quantities.<Name><T>.Create(<invariant literal>). Create(T) exists on Mass<T> and Acceleration1D<T> in ktsu.Semantics.Quantities 5.12.1 and takes the SI base unit, which matches what C++ assigns.
• Vector quantities (e.g. Velocity3D): put the value in every component, as the C++ side does. The type exposes X/Y/Z, One and From<Unit>(x, y, z); confirm which construction route compiles.
• Structs that travel as bytes (TravelsAsBytes) still get their parameterless constructor.
• Extend TestSemanticVectorAndColourMembersStartAtTheirDefaults in CodeGenerationRoundTripTests to cover one magnitude quantity and one vector quantity, and assert the runtime starting value.