= Solution
Curvature invariants and the electromagnetic invariant diverge at $r=0$, so it is a genuine <curvature singularity>, not a coordinate singularity. The causal structures are the higher-dimensional analogues of the familiar four-dimensional diagrams:
* for $r_-=0$, there is one nondegenerate horizon and a spacelike singularity, giving the Schwarzschild Penrose diagram;
* for $0<r_-<r_+$, the outer event horizon and inner <Cauchy horizon> enclose a region ending at a timelike singularity, with the maximally extended Reissner-Nordström chain of blocks;
* for $r_-=r_+$, the horizon is degenerate with $\kappa=0$, the singularity remains timelike, and the extremal throat has infinite spatial length.
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