guard negative operands in MathLib::value shift operators #20000

Job	Run time
build_cmake (ubuntu:24.04, true, true)	2m 24s
build_cmake (ubuntu:25.10, true, true)	6m 0s
build_make (ubuntu:25.10)	4m 55s
build_make (ubuntu:24.04)	3m 39s
build_cmake (alpine:3.23, false, false)	4m 8s
build_make (alpine:3.23)	5m 53s
	26m 59s

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