Streaming p × r cross-moment accumulator for the closed-form polar frame
update (issue #972). Sums decoder-target outer products Σ_i t_i c_iᵀ
(ambient target t_i ∈ ℝ^p against in-span coordinate c_i ∈ ℝ^r) so the
frame can be re-polared from accumulated evidence WITHOUT re-touching the
border. Accumulation is O(p r) per update and never forms a p × p matrix.
Verification helper (issue #972): the factored arrow-Schur border dimension
equals Σ_k M_k · r_k exactly. Returns Ok(()) iff the invariant holds for
term, else an explanatory error. Compiled-in so the border-size contract is
checkable at runtime, not only in tests.
Verification helper (issue #972): recover the planted low-rank column span of
an atom by polaring the decoder-target cross-moment and report the largest
principal angle (radians) between the recovered frame and a planted
orthonormal frame planted (p × r).