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CHANGELOG: seal the 2026.9.18.1 section #2007

Job	Run time
examples: the curriculum builds	6m 45s
integration: mcpp builds & runs xlings	10m 3s
build + unit tests (linux x86_64, self-host)	18m 47s
toolchain: gcc (cold self-host)	8m 12s
toolchain: musl + llvm	11m 26s
	55m 13s