Exact: add a rational Jordan geometry oracle

[LightChainr]

Summary

• certify an explicit rational 2×2 size-two Jordan block via its repeated eigenvalue and nonzero square-zero nilpotent part
• verify trace, determinant, discriminant, and Jordan structure under a nontrivial similarity transform
• verify the exact cocycle law C(a)C(b) = C(a+b) for a square-zero generator
• expose a rational coalescing family whose discriminant and eigenvector angle vanish monotonically
• state explicitly that this synthetic oracle makes no empirical transfer-matrix or norm-4 conclusion

Tests

• python -m unittest tests.test_jordan_geometry_oracle (6 passed)
• python -m unittest discover -s tests -p 'test_*.py' (806 passed, 1 skipped)
• CLI contract validation and Python compilation

Advances #218. The parent issue remains open because no measured transfer matrix, covariance estimate, fitted Jordan block, physical logarithmic pair, or norm-4 conclusion is supplied.