Instruments _anchor_pull to measure versor_condition(pulled_F) before
unitize_versor across 4,138 samples from session/chat test suites.
Verdict: (c) upstream construction violation. _slerp_toward operates on
S^31 (the 32D unit sphere) rather than the Spin sub-manifold, producing
off-manifold state with vc up to 38.58 for non-negligible field-to-anchor
angles. Distribution is strictly bimodal: vc < 1e-6 when theta ≈ 0 (slerp
is near-identity), otherwise vc >> 1e-3 — confirming the slerp is the
sole source.
Recommended fix (separate PR): replace _slerp_toward with rotor geodesic
interpolation via the Lie group exponential map (same principle as
rotor_power used in generate/stream.py:220), eliminating the post-slerp
unitize by construction.
