Fix intermittent test failures on Windows CI due to Tcl/Tk initialization race condition #558
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Plot tests on Windows CI were failing intermittently with
_tkinter.TclError: Can't find a usable init.tcl, causing flaky test runs that would pass on retry. This was a race condition where uv's Python extraction on Windows didn't complete before matplotlib tried to initialize Tkinter.This fix sets
MPLBACKEND=Aggin the test workflow, which tells matplotlib to use the non-interactive Agg backend instead of Tkinter. This is standard practice for CI environments and eliminates the Tcl dependency entirely. Plot functionality remains fully tested—the Agg backend generates static images instead of interactive windows.