Consider a release policy for edge geometry built for snapping

Feature

#13789 builds edge geometry on demand and retains it after a model participates in a snap pass. This avoids repeated rebuilds, but applications that only snap occasionally may retain geometry they no longer need.

Explore reclaiming that geometry while edges are hidden. A short expiry could cause repeated rebuilds between normal pauses in cursor movement, so compare memory savings against rebuild cost before choosing a policy.

Derived edge buffers currently aren't included in Cesium3DTileset.totalMemoryUsageInBytes, so the tileset cache budget doesn't account for this memory either.

Raised by @danielzhong in https://github.com/CesiumGS/cesium/pull/13789#discussion_r3982944984.

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