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2026.9.5.3: a feature-selected rule collection, mcpp.rules.* naming, and a variant switch the fast path no longer replays #304

Job	Run time
build 3 targets on windows	6m 47s
build 3 targets on linux	4m 44s
openkal e2e (the scripts, on a runner that has llvm)	6m 29s
build 3 targets on macos	4m 49s
run 3 builds on linux	6s
run 3 builds on windows	7s
run 3 builds on macos	6s
	23m 8s