Fix "dead state tree node" error by creating snapshots of parent region for block calculations

[cmdcolin]

I found a relatively reproducible procedure to create the dead state tree node error, and a probable fix for this particular case

• Go to LGV import form
• Enter EDEN
• Select EDEN from the dropdown
• Hit open

A good portion of the time, this produces the error in #3181

I found that the "parentRegion" attribute in the static/dynamic blocks were not snapshots and were instead proxies for the state tree node, creating this error

Note that this change could mildly slow down the block calculations. Note also that parentRegion is not even used anywhere in our codebase, but have some hypothetical use cases

Fixes #3181

[cmdcolin]

an alternative fix could consider removing parentRegion

[codecov]

Codecov Report

Merging #3182 (9a470f2) into main (c8c79c4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3182      +/-   ##
==========================================
+ Coverage   59.49%   59.52%   +0.02%     
==========================================
  Files         671      671              
  Lines       28644    28646       +2     
  Branches     6934     6936       +2     
==========================================
+ Hits        17043    17052       +9     
+ Misses      11279    11272       -7     
  Partials      322      322              

Impacted Files	Coverage Δ
packages/core/util/calculateDynamicBlocks.ts	96.36% <100.00%> (+0.06%)	⬆️
packages/core/util/calculateStaticBlocks.ts	100.00% <100.00%> (ø)
products/jbrowse-web/src/util.ts	48.33% <0.00%> (ø)
packages/core/util/layouts/GranularRectLayout.ts	87.28% <0.00%> (+0.42%)	⬆️
...FeatureRenderer/components/ProcessedTranscript.tsx	88.73% <0.00%> (+1.40%)	⬆️
...gins/svg/src/SvgFeatureRenderer/components/util.ts	95.91% <0.00%> (+4.08%)	⬆️
...src/SvgFeatureRenderer/components/FeatureLabel.tsx	86.11% <0.00%> (+5.55%)	⬆️
...svg/src/SvgFeatureRenderer/components/Segments.tsx	92.85% <0.00%> (+7.14%)	⬆️

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[cmdcolin]

maybe can merge for now