ci: add cross-platform install matrix and API smoke checks #7

Job	Run time
Install pep517 \| macos-latest \| py3.11	20s
Install pep517 \| ubuntu-latest \| py3.11	19s
Install pep517 \| macos-latest \| py3.12	27s
Install fallback \| ubuntu-latest \| py3.11	21s
Install fallback \| windows-latest \| py3.12	49s
Install pep517 \| windows-latest \| py3.12	56s
Install fallback \| macos-latest \| py3.11	19s
Install pep517 \| ubuntu-latest \| py3.12	24s
Install fallback \| macos-latest \| py3.12	33s
Install fallback \| ubuntu-latest \| py3.12	17s
Install fallback \| windows-latest \| py3.11	51s
Install pep517 \| windows-latest \| py3.11	53s
API smoke \| macos-latest \| py3.11	24s
API smoke \| windows-latest \| py3.12	1m 5s
API smoke \| windows-latest \| py3.11	1m 31s
API smoke \| ubuntu-latest \| py3.11	28s
API smoke \| ubuntu-latest \| py3.12	35s
API smoke \| macos-latest \| py3.12	27s
Live Claude + Haiku validation (manual)	0s
	10m 59s

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