[compiler] Phase 8: Add multi-error test fixture and update plan (#35877)

Add test fixture demonstrating fault tolerance: the compiler now reports
both a mutation error and a ref access error in the same function, where
previously only one would be reported before bailing out.

Update plan doc to mark all phases as complete.

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[//]: # (BEGIN SAPLING FOOTER)
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* #35888
* #35884
* #35883
* #35882
* #35881
* #35880
* #35879
* #35878
* __->__ #35877