[ScottishTapWater]

Summary

• Add addition, subtraction, and unary negation directly to Quantity<TDimension>.
• Preserve dimensions through those expressions and inferred local initialisers so subsequent multiplication and division can be generated.
• Keep compiler-bound custom operator return types authoritative, with regression coverage.

Verification

• Tests written before implementation.
• All 452 tests pass: 315 core, 136 Roslyn, and 1 packaged-consumer integration test.
• Formatting and diff checks pass (excluding the existing intentional RS1041 warning for the .NET 10 generator).
• Independent production and test reviews completed, then repeated after fixes; no outstanding findings.
• Explicit test-ownership and class-responsibility reviews completed.

Same-dimension operators live in the core generic type; no additional generated overloads are required.