ci: add cross-platform install matrix and API smoke checks #6

Job	Run time
Install fallback \| ubuntu-latest \| py3.12	19s
Install fallback \| macos-latest \| py3.12	21s
Install pep517 \| windows-latest \| py3.12	51s
Install fallback \| ubuntu-latest \| py3.11	19s
Install pep517 \| macos-latest \| py3.11	27s
Install pep517 \| ubuntu-latest \| py3.11	17s
Install pep517 \| macos-latest \| py3.12	30s
Install pep517 \| windows-latest \| py3.11	54s
Install fallback \| windows-latest \| py3.11	1m 16s
Install fallback \| macos-latest \| py3.11	18s
Install fallback \| windows-latest \| py3.12	57s
Install pep517 \| ubuntu-latest \| py3.12	21s
API smoke \| macos-latest \| py3.12	24s
API smoke \| ubuntu-latest \| py3.12	31s
API smoke \| ubuntu-latest \| py3.11	31s
API smoke \| macos-latest \| py3.11	26s
API smoke \| windows-latest \| py3.11	2m 2s
API smoke \| windows-latest \| py3.12	1m 40s
Live Claude + Haiku validation (manual)	0s
	12m 24s

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