fix(react): ensure edges are materialized when React runs effects

[bowheart]

Description

The changes in #154 ensured that edges recreate themselves if other components destroy them when React is running effects, but I missed two things:

• The _prevEdge references are not cleared when we run a cleanup cycle.
• Edges that are destroyed by other component effects and recreated don't have isMaterialized: true.

I actually didn't miss these, but I missed the fact that together, they mean that components added later will continue adding to the existing singly-linked list chain and when later cleanup cycles walk up that chain, they'll see non-materialized nodes.

The fix is easy: Either break the chain when a cleanup cycle runs (by clearing _prevEdge references) or always create pre-materialized nodes in useEffects. The former is risker - it may still be vulnerable to who-knows-what React rendering sequences, and the weak ref chain cleans itself up anyway - so do the latter.

Issues

Resolves #165