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A headless Mac or asleep built-in panel leaves ScreenCaptureKit with 0 shareable displays while TCC grants pass — health_report stays ok and every capture silently returns 0x0 (#67165). Guard at the report seam (_apply_display_count_guard, both real and fallback paths): flips the screen_capture_capability check to fail with recovery actions (wake display / HDMI dummy / virtual display) and downgrades ok -> degraded. The empty-discovery reason ladder gains the matching darwin rung. Composed from #52949 (sujeet111) and #67259 (webtecnica); both PRs predate the doctor rewrite and the envelope normalization on main, so this reimplements their shared intent at the current seams. Co-authored-by: Sujeet <64351924+sujeet111@users.noreply.github.com> Co-authored-by: webtecnica <75556242+webtecnica@users.noreply.github.com>