feat(examples): validate galgebra against Goyder cheat sheet (#506)

[utiberious]

Closes #506

Adds examples/Terminal/cheatsheet_validation.py validating galgebra against Russell Goyder's geometric algebra cheat sheet, covering all 7 sections:

1. geometric product decomposition: ab = a·b + a∧b, symmetric/antisymmetric parts
2. vector-multivector grade raising/lowering: a*B = a<B + a∧B
3. bivector identities: a·(b∧c) = (a·b)c - (a·c)b, (a∧b)·(c∧d) scalar formula
4. commutator product: B×v = B·v, grade-preserving on bivectors, Jacobi identity
5. pseudoscalar: I²=-1, centrality in 3D, dual via I⁻¹
6. outermorphism: rotor normalization, rotation sandwich, det(rotation)=1
7. adjoint: a·F̄(b) = F(a)·b, det(FG) = det(F)·det(G)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.32%. Comparing base (a860be1) to head (69f21db).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #557      +/-   ##
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+ Coverage   73.11%   73.32%   +0.21%     
==========================================
  Files          22       22              
  Lines        4475     4458      -17     
==========================================
- Hits         3272     3269       -3     
+ Misses       1203     1189      -14     

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[utensil]

Thanks for the cheatsheet validation script. Math across all 7 sections checks out — geometric product decomposition, vector-multivector products, bivector contraction identities, commutator/Jacobi identity, pseudoscalar properties, rotation outermorphism, and adjoint definition all verify correctly in G(3,0).

One significant issue: examples/Terminal/cheatsheet_validation.py is not collected by pytest. The CI config (setup.cfg: python_files = test_*.py) only collects files matching test_*.py. The pytest invocation covers test/ and nbval on examples/ipython|primer/ — examples/Terminal/ is not included. The codecov/patch passes vacuously because --cov=galgebra only measures galgebra/ source coverage, and this PR doesn't touch any galgebra source. The script runs zero assertions during CI.

Targeted tests belong in test/. Please move this to test/test_cheatsheet.py so the assertions actually run in CI. Notebooks in examples/ are fine as examples that happen to improve coverage, but new targeted tests go in test/.

Minor: section 5 uses the Iinv+ dual convention (dual(A) = A * I^{-1}) computed manually via I.inv(). That's mathematically correct, but galgebra's built-in dual() defaults to I+ (dual(A) = I*A). Worth a comment so readers don't assume dual() behaves this way.