Interface ExportSupportProbeOptions

Also DRY-RUN the active configuration: instantiate a real VideoEncoder with the config the fallback ladder selected, encode a handful of synthetic frames at the export size, and verify encoded chunks actually come out. isConfigSupported is only the browser's prediction — a dry run catches encoders that claim support but fail or hang once configured (the failure class export crash reports see in the wild). Costs a real encode of ~8 frames (typically well under a second; bounded by an internal timeout), so it defaults to off. When requested, the overall supported flag reflects the dry-run outcome.