#4271 refactor(apple): name the foreground reading type

This PR applies the optional items from the #4267 review (https://github.com/callstack/agent-device/pull/4267#issuecomment-6684098121). It does not change behavior.

• system-surface-foreground.ts exports SystemSurfaceForegroundReading. The probe signature and both snapshot-route.ts sites use it in place of Awaited<ReturnType<SystemSurfaceForegroundProbe>>.
• The native wedge test block has a comment that says it pins the stale abandon stamp. Its expectation is renamed to secondAbandoned, because the expectation waits for the abandonment, not for the read to drain.

Validation: tested at f9869c44a7. The check:affected receipt and the native run follow in a comment.

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