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>
Teknium