When every DRM card's vendor file or every Windows display adapter's
MatchingDeviceId was unreadable, classify_gpu_vendors returned 'none' and
availability refused with unsupported_gpu, contradicting the documented
'unreadable GPU passes' rule. Windows now records an unreadable numbered
adapter as an empty id so the classifier sees it.
Also drops the type-narrowing assert in availability() and makes
_action's gpu_label required.
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