[DX23876]

AutoWorkoutCard runs its scan from a .task on a Group whose only child is the card, shown once a candidate exists. SwiftUI never runs a .task attached to a Group with no child, so the scan never starts and the prompt never appears on any Today screen, even with "Auto-detect workouts" switched on. Same root cause as #2765 (journal card).

A placeholder view would start the task but is no fix here: whenever the detector finds nothing it would sit in the Today stack and add its spacing. The scan now runs in AutoWorkoutCandidateLoader, an ObservableObject the card's body hands the current key (refresh sequence, toggle). It ignores an unchanged key and publishes from its task, never during the view update. Save, dismiss and the "new window resets the session guard" rule behave as before.

Verification

• A standalone SwiftUI check on macOS: .task on a Group (or an EmptyView) with no child never runs; an empty VStack or a zero-size view runs it but adds the stack's spacing.
• The same check with this loader pattern: the load starts, the card appears once a candidate arrives (stack 32 → 94 pt), and with no candidate the stack keeps its height (32 pt, no gap).
• macOS Strand and iOS NOOPiOS build.
• Not seen with a real candidate on a device: the demo data has no window the detector picks up.