Position3D.Cross(Force3D) and Position3D.Cross(Velocity3D) don't compile: vector overloads don't forward their base's typed cross products #506

What's wrong

#456 made semantic overloads forward their base type's relationship operators, through ForwardedOperators in QuantitiesGenerator.cs. It did not forward the typed Cross and Dot methods. Those are emitted only for base vector types, from emission.ProductsByOwner (around QuantitiesGenerator.cs:1219-1241), and EmitVectorOverloadType never emits them.

They are instance methods on a readonly record struct. Neither inheritance nor implicit widening can reach them through the receiver.

Position3D is an overload of Displacement3D ("Location in 3D space relative to an origin"), so it is the natural r. Yet both cross products declared on Length are missing from it:

• cross(Displacement3D, Force3D) -> Torque3D
• cross(Displacement3D, Velocity3D) -> SpecificAngularMomentum3D

The generated Position3D.g.cs has only Dot(Position3D) and Cross(Position3D). Displacement3D.g.cs:209 has Torque3D<T> Cross(Force3D<T> other).

Reproduction (net10.0, at be8aa23)

Position3D<double> r = Position3D<double>.FromMeter(0.5, 0, 0);
Force3D<double> f = Force3D<double>.FromNewton(0, 10, 0);
Velocity3D<double> v = Velocity3D<double>.FromMeterPerSecond(0, 10, 0);

var tau = r.Cross(f); // CS1503: cannot convert Force3D<double> to Position3D<double>
var h   = r.Cross(v); // CS1503

Today callers have to write ((Displacement3D<double>)r).Cross(f). Translation3D has the same gap, and so will any future vector overload of a dimension that has typed products. An overload on the argument side already works, because the argument widens.

Suggested fix / acceptance criteria

• EmitVectorOverloadType forwards every entry of ProductsByOwner[baseTypeName] as => ((Base<T>)this).Cross(other), and does the same for Dot if a typed dot exists. Results stay base types, as Physics operators are unreachable when a semantic overload is the left operand: Distance / Duration, Width * ForceMagnitude, Weight * Length do not compile #456 did for operators.
• Position3D.Cross(Force3D) returns Torque3D with the sign of r × F: +0.5 x̂ × +10 ŷ = +5 ẑ.
• Position3D.Cross(Velocity3D) returns SpecificAngularMomentum3D.
• Translation3D gets the same methods.
• GeneratorOutputInvariantTests (the Physics operators are unreachable when a semantic overload is the left operand: Distance / Duration, Width * ForceMagnitude, Weight * Length do not compile #456 "forwards its base's products" check) covers typed product methods as well as operators.