rotor_power previously returned the IDENTITY for any non-simple rotor — an
approximation where exactness was available (Pillar II, Semantic Rigor) that
silently collapsed geodesic interpolation (slerp / supervised blend) to a
no-op while closure stayed green. Replace the identity-fallback with the
invariant (bivector) decomposition: a general Cl(4,1) rotor factors into two
commuting simple rotors R = R1 R2, so R^a = R1^a R2^a exactly, each via the
existing simple closed form, with a dedicated closed form for the isoclinic
(coincident-plane) case. Built from the geometric product alone — no scipy,
no GA library (Pillar III, Third Door); exact f64 on the existing product
table (Pillar I, Mechanical Sympathy).
- Simple path is byte-identical (0.0 delta); 66 existing algebra tests pass.
- tests/test_rotor_power_general.py pins R^1=R, (R^.5)^2=R, R^a R^b=R^(a+b),
R^0=1, closure, isoclinic, and replay determinism across every plane type
incl e5 boosts (441 pass), to machine precision (<= 6.5e-10).
This is the substrate cause of the Third-Door blend degeneration (fidelity
finding #1, issues #16/#18): with a real rotor_power, supervised_blend and
dual_correction_slerp now interpolate monotonically and land on target. Once
merged, the ADR-0239 blend xfail on #15 flips green.
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