dispatch's call_failure and its except clause resolved the module globals at
call time, so a program rebinding __main__._BindingRejection = ValueError let
the internal marker type leak into model code. Bind _RuntimeError_cls and
_BindingRejection_cls into _run locals before the program runs (names distinct
from the module globals so the assignment RHS resolves the global, not an
unbound local); dispatch now uses the locals. A regression test rebinds
_BindingRejection and asserts a host rejection still surfaces as RuntimeError.
The sys.__stdout__ flush test now reconfigures the streams back to block
buffering (write_through=False) so the settlement drain path is what the case
pins — verified fail-before: binding the stream objects instead of their flush
methods turns the test red.
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