docs+ci: 本地上手三步走 + 平台矩阵拆分(linux/macos/win10/win11/win 无 MSVC STL) #39

Job	Run time
exercises ↔ solutions filename parity	5s
validate (windows-11, no MSVC STL)	8m 15s
validate (macos)	2m 32s
validate (windows-11 baseline)	6m 21s
validate (linux)	5m 27s
validate (windows-10 baseline)	7m 5s
	29m 45s

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