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Easier than previous commit message seemed to think.

Since we already have finishCallback functions in play in any scene
where maybes can also be in play, we can just make those responsible
for copying out a pointer from the child assembler to the parent value.
That makes the child assembler able to create a new value late in
its lifecycle and still expect it can land in a proper home.

Tests needed badly; this is *oodles* of edge cases.
Unrelated fix needed first.  (Might make that elsewhere and rebase
this to include that, to reduce the carnage ever so slightly.)