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feat: date-based version scheme + xlings pin single source of truth + fix #291 (2026.7.27.1) #574

Job	Run time
toolchain: gcc (cold self-host)	5m 50s
toolchain: musl + llvm	3m 15s
integration: mcpp builds & runs xlings	6m 36s
build + unit tests (linux x86_64, self-host)	7m 27s
toolchain: musl + llvm	6m 7s
toolchain: gcc (cold self-host)	5m 50s
integration: mcpp builds & runs xlings	6m 36s
build + unit tests (linux x86_64, self-host)	7m 27s
	49m 8s