Resetting Folder.current for the next run of a named engine now drops the
last run's uncaught exception and $.error first, so memory only they held
no longer makes the reset fail with the heap limit on every later run.
A File an earlier run left open now counts as closed in a later run:
setting its encoding stores it for the next open, and reading the encoding
gives the stored one.
docs/mcp.md: engines stay on the thread that made them, and a
#targetengine line in a file $.evalFile runs makes that call throw a
SyntaxError when it runs.
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