Test the A.4.27 signed subset expansion

[utiberious]

Refs #16.

This adds a concrete Cl3 test for Appendix A.4.27 of arXiv:1205.5935. It uses B₂ = e₁ ∧ e₂ + 2(e₂ ∧ e₃), a₁ = e₁ + e₂, a₂ = e₂ + e₃, and a₃ = e₃ + e₁, then verifies the +, -, + two-element-subset expansion for B₂ ⨼ (a₁ ∧ a₂ ∧ a₃).

The left-hand side and all three summands are nonzero, and the test checks their exact components. The required sign pattern is unique for this witness: each of the other seven sign choices produces a different result.

PR #18 adds @test to three expressions already present in the suite. This patch covers the separate A.4.27 gap, so the changes are complementary.

Tested with Julia 1.10.11 and Python galgebra==0.6.0:

julia --project=. -e 'using Pkg; Pkg.test()'

Result: 1,340 passed, 1 pre-existing broken, 1,341 total.