Zig: Fix RunAfter behavior on device suspension

[keeshux]

Rather than polling every 10ms:

• First, use absolute deadlines rather than deltas
• Schedule wake-up at the next earliest callback deadline
• If a new callback is scheduled earlier, interrupt the pending wake-up and reschedule with the new deadline

For this to work, replace the naïve sleepMs() spin-lock with platform-specific waitUntil() conditions.