Vector0 quantities do not enforce the non-negativity invariant

[matt-edmondson]

Summary

docs/strategy-unified-vector-quantities.md defines IVector0<TSelf, T> as a magnitude-only quantity that "enforces non-negativity as a type-level invariant", citing examples like:

Speed<double> s = Speed.FromMetersPerSecond(-1.0);   // throws: magnitude cannot be negative

In practice, no enforcement exists. Generated Vector0 quantities (Speed, Mass, Distance, Energy, Area, …) accept any value, including negatives.

Evidence

Semantics.Quantities/Generated/.../Mass.g.cs:

public static Mass<T> FromKilogram(T value) => Create(value);

No guard. grep -E "cannot be negative|ArgumentOutOfRange|absolute zero" Generated/.../{Speed,Mass,Temperature}.g.cs returns nothing.

Semantics.Quantities/PhysicalQuantity.cs also does not validate; it is a thin wrapper exposing Value.

Suggested next step

Decide where the constraint lives:

1. Emit a guard inside the Create / From* factories of any quantity declared as Vector0 in dimensions.json, throwing ArgumentException (per CLAUDE.md's validation guidance) when T.Sign(value) < 0.
2. Or move the guard into a shared helper on PhysicalQuantity<T> / a Vector0<T> base.

Either way, add tests in Semantics.Test that assert negative-input rejection across every Vector0 quantity.

Area / Severity

SourceGenerators / Quantities · incomplete

[matt-edmondson]

Resolved by #68 (commit 039afc4). Vector0Guards.EnsureNonNegative is invoked from every generated From{Unit} factory on V0 base types and V0 semantic overloads, throwing ArgumentException for negative inputs. Coverage is in Semantics.Test/Quantities/Vector0InvariantTests.cs (Speed, Mass, Length, Energy, Distance, Weight, plus float/decimal storage variants). The committed Generated/ tree was stale relative to that work and is being refreshed in #70.

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