test(mv): expand and reorganize is_blade coverage (#537)

[utiberious]

Summary

Replaces test_is_blade_null with focused methods inside TestMv, grouped by comment, covering every branch of is_blade() and the null-blade guards added in the same fix:

Test	Branch
test_is_blade_zero_and_scalar	is_zero() path and grade-0 scalar
test_is_blade_vectors	null and non-null grade-1
test_is_blade_bivectors	null and non-null grade-2
test_is_blade_trivector	grade-3 via is_versor()
test_is_blade_grade3_known_limitation	DFM counterexample (R^6, no coords, 0.35s); documents B^B=0 false positive for grade >= 3
test_is_blade_non_homogeneous	i_grade is None path
test_is_blade_result_cached	blade_flg cache path
test_reflect_in_null_blade_raises	null-blade guard in reflect_in_blade()
test_project_in_null_blade_raises	null-blade guard in project_in_blade()

Also registers slow pytest marker in setup.cfg.

[utensil]

Thanks for the expansion. The slow marker is registered in setup.cfg but not applied to any test — it should be on test_is_blade_grade3_known_limitation (the R⁶ case).

Also missing a test for (self ^ self).is_zero() returning False — a non-blade correctly rejected. Something like e1^e2 + e3^e4 in R⁴ would cover that branch.

[utensil]

Thanks, merging.