ci: add cross-platform install matrix and API smoke checks #5

Job	Run time
Install fallback \| ubuntu-latest \| py3.12	21s
Install pep517 \| macos-latest \| py3.11	19s
Install pep517 \| ubuntu-latest \| py3.12	22s
Install fallback \| macos-latest \| py3.12	17s
Install fallback \| ubuntu-latest \| py3.11	21s
Install pep517 \| windows-latest \| py3.11	59s
Install pep517 \| ubuntu-latest \| py3.11	20s
Install fallback \| macos-latest \| py3.11	19s
Install fallback \| windows-latest \| py3.12	1m 31s
Install pep517 \| windows-latest \| py3.12	1m 3s
Install pep517 \| macos-latest \| py3.12	20s
Install fallback \| windows-latest \| py3.11	1m 24s
API smoke \| windows-latest \| py3.11	1m 2s
API smoke \| ubuntu-latest \| py3.11	28s
API smoke \| windows-latest \| py3.12	1m 12s
API smoke \| macos-latest \| py3.11	26s
API smoke \| ubuntu-latest \| py3.12	35s
API smoke \| macos-latest \| py3.12	23s
Live Claude + Haiku validation (manual)	0s
	11m 42s

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