Vector0 magnitudes go negative through unary minus and scalar * and /: `-Temperature.FromKelvin(300)` is −300 K

[matt-edmondson]

What's wrong

The quantities generator deliberately withholds binary - for V0 forms so that magnitudes stay non-negative (QuantitiesGenerator.cs:843-847, if (!isV0)). But for every scalar form, V0 included, it still emits:

• unary operator -(value) => Create(-value.Quantity) (lines ~849-857)
• operator *(q, T) and operator *(T, q)
• operator /(q, T)

Create intentionally does not guard (CLAUDE.md ~l.84), and #50 added Vector0Guards only to the factories. So a V0 quantity becomes negative through a single operator.

Failure scenario

Reproduced:

• -Temperature<double>.FromKelvin(300) gives −300 K, below absolute zero.
• -Speed<double>.FromMeterPerSecond(3) gives −3 m/s for a magnitude.
• Speed<double>.FromMeterPerSecond(2) * -3.0 gives −6.

This contradicts docs/architecture.md:400 and CLAUDE.md rule 4 ("Physical constraints are enforced structurally via the V0 form"). docs/complete-library-guide.md:137 claims Speed.Create(-1) throws, but it returns −1. Downstream code that relies on the type guaranteeing non-negativity (square roots, logs, dividing by magnitudes) gets NaN or wrong signs.

Suggested fix / acceptance criteria

• For V0 forms, don't emit unary negation (as is already done for binary -).
• For V0 forms, guard scalar * and / with Vector0Guards.EnsureNonNegative, or restrict them to non-negative scalars.
• Fix the complete-library-guide.md line (or make Create behave as it says).
• Tests: unary negation of a V0 is unavailable, or throws. Scaling a V0 by a negative number throws.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. This breaks the library's core structural invariant: V0 magnitudes should never be negative. A single operator gets a V0 quantity past the guards added in Vector0 quantities do not enforce the non-negativity invariant #50, and downstream sqrt, log, or division then produces NaN or the wrong sign. It takes an explicit negation or a negative scalar to trigger, which is why this isn't rated higher.
• Area / suggested owner: Source generator, QuantitiesGenerator.cs (the operator emission around l.843–857). Also a docs fix in docs/complete-library-guide.md:137.
• Duplicates / in progress: None. It follows up Vector0 quantities do not enforce the non-negativity invariant #50, which guarded only the factories. TemperatureDelta/Rise/Drop.FromCelsius(10) is 283.15 K: the absolute-scale offset is applied to a temperature difference #283 is in the same generator and could share a PR. No open PR (Bump the ktsu group with 15 updates #278 is only a dependency bump).
• Next step: Skip unary - for V0, the same way binary - is already skipped. Guard scalar * and / with Vector0Guards.EnsureNonNegative. Correct the guide's claim about Create. Add tests that V0 negation is unavailable and that scaling a V0 by a negative number throws.

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