Solve min 1/2 y' H y - rhs' y s.t. ||y||_2 <= M (KKT secular equation).

The proper ridge-ball solution y = (H + nu I)^{-1} rhs with nu >= 0 chosen so ||y|| = M (or nu = 0 if the unconstrained solution is interior). This is the H-metric projection, not a naive Euclidean radial clip; used by both the estimator and the constrained bootstrap.

Usage

ridge_ball_solve(H, rhs, M)