feat: add solid sphere surface

[Robby955]

Adds the solid sphere (closed unit ball) to the spatial surfaces API in PhyslibAlpha, following the merged Line.lean: the embedding into Space d.succ, the associated measure (ambient volume restricted to the closed unit ball), the temperate-growth instance, and the distribution. A solid region has positive ambient volume, so the measure-zero requirement does not apply and is replaced by a positive-volume statement. Part of #943.

[github-actions]

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[jstoobysmith]

Looks good - approved. Will merge shortly.