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The engine half of #634: conditional dependency replacement, manifest identity, staged Android and Mach-O closures, runner runtime files, the resolved graph (2026.9.14.2) #1943

Job	Run time
build + unit tests (linux x86_64, self-host)	7m 30s
integration: mcpp builds & runs xlings	7m 31s
examples: the curriculum builds	7m 23s
toolchain: musl + llvm	7m 29s
toolchain: gcc (cold self-host)	7m 29s
	37m 22s