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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) #1944

Job	Run time
integration: mcpp builds & runs xlings	6m 4s
toolchain: gcc (cold self-host)	1m 49s
examples: the curriculum builds	1m 37s
build + unit tests (linux x86_64, self-host)	3m 3s
toolchain: musl + llvm	6m 2s
	18m 35s