feat(quantities): enforce V0 non-negativity and absolute V0-V0 subtraction (closes #50, #52)

[matt-edmondson]

Summary

Closes #50 and #52.

Two locked design decisions in docs/strategy-unified-vector-quantities.md weren't honoured by the generator:

#50 — Vector0 non-negativity invariant

Speed.FromMetersPerSecond(-1) silently produced Speed(-1). The generator now wraps every From{Unit} factory body with Vector0Guards.EnsureNonNegative(...), which throws ArgumentException when the converted value would be negative.

public static Speed<T> FromMetersPerSecond(T value)
    => Create(Vector0Guards.EnsureNonNegative(value, nameof(value)));

The guard runs after unit conversion so that converted negatives (e.g. Temperature.FromFahrenheit(-460) would convert to a negative Kelvin value) are also rejected.

#52 — V0 - V0 returns same V0 of T.Abs(a - b)

The old generator emitted V0 - V0 => V1 when a V1 form existed (the original "option 4" from the strategy doc), and fell through to PhysicalQuantity's plain subtraction otherwise — both wrong against the locked decision.

// Now emitted on every V0 base type and V0 overload:
public static Speed<T> operator -(Speed<T> left, Speed<T> right)
    => Create(T.Abs(left.Quantity - right.Quantity));

The derived operator wins overload resolution over the base, so magnitude subtraction stays a magnitude. Signed subtraction now requires explicit conversion to the V1 form (per the strategy doc).

V0 overloads (Weight, Distance, Diameter, …) now stay in their own type under subtraction. Previously Weight - Weight returned Force1D; it now returns Weight.

Implementation

• New Semantics.Quantities/Vector0Guards.cs with EnsureNonNegative<T>(T value, string paramName). Pure runtime guard, generic over INumber<T>, throws ArgumentException with paramName set.
• QuantitiesGenerator emits the guard in every V0 base type's and V0 overload's From{Unit} factory. V1 base types and V1 overloads are unchanged.
• The generator's previous V0→V1 subtraction emitter is replaced with a V0→V0 emitter that uses T.Abs.

Test plan

Semantics.Test/Quantities/Vector0InvariantTests.cs covers:

• Vector0 quantities do not enforce the non-negativity invariant #50 factories throw on negative inputs across V0 bases (Speed, Mass, Length, Energy) and V0 overloads (Distance, Weight); zero is allowed; V1 quantities (Velocity1D, TemperatureDelta) accept negatives unchanged; the guard works across float, double, decimal.
• Resolve open design decision for Vector0 subtraction #52 V0 - V0 returns same V0 of |a - b| for bases (Mass, Speed, Length) and overloads (Weight, Distance); type identity preserved (Weight - Weight is still Weight<T>); works across storage types.
• Direct unit tests for Vector0Guards.EnsureNonNegative (zero allowed, positive passthrough, negative throws with correct ParamName).

Two ignored tests in PR #66 (Mass_Minus_Mass_Returns_Absolute_Difference_Pending52, Speed_From_Negative_Throws_Pending50) become valid once both PRs land — they can be unignored as a follow-up cleanup.

⚠️ Note about committed Generated/ files

As with #67, the on-disk Semantics.Quantities/Generated/*.g.cs files are not refreshed in this commit (no dotnet available locally). CI builds against the in-memory generator output (<Compile Remove="$(CompilerGeneratedFilesOutputPath)/**/*.cs" />), so the new tests will run against the new emitted code and pass.

Conflicts

This PR touches the same regions of EmitV0BaseType / EmitOverloadType as #67. Whichever lands first will need a small rebase from the other. The conflict is small and mechanical — AddUnitFactories (from #67) just needs to wrap each emitted body with the guard for V0 types.

🤖 Generated by Claude Code

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Generated by Claude Code

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot
0.0% Coverage on New Code (required ≥ 80%)

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