ci: add cross-platform install matrix and API smoke checks #4

Job	Run time
Install fallback \| ubuntu-latest \| py3.12	19s
Install pep517 \| macos-latest \| py3.11	17s
Install fallback \| macos-latest \| py3.11	23s
Install fallback \| ubuntu-latest \| py3.11	21s
Install pep517 \| windows-latest \| py3.11	1m 1s
Install pep517 \| ubuntu-latest \| py3.11	16s
Install pep517 \| ubuntu-latest \| py3.12	27s
Install fallback \| macos-latest \| py3.12	23s
Install fallback \| windows-latest \| py3.12	53s
Install pep517 \| windows-latest \| py3.12	54s
Install fallback \| windows-latest \| py3.11	1m 1s
Install pep517 \| macos-latest \| py3.12	17s
API smoke \| macos-latest \| py3.12	40s
API smoke \| windows-latest \| py3.11	2m 19s
API smoke \| windows-latest \| py3.12	1m 21s
API smoke \| macos-latest \| py3.11	21s
API smoke \| ubuntu-latest \| py3.11	22s
API smoke \| ubuntu-latest \| py3.12	24s
Live Claude + Haiku validation (manual)	0s
	11m 59s

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