Fix live scanner freezing/stutter and clean up detection overlay (iOS + Android)

[komodgn]

Summary

Fixes several issues in the live clover scanner across both iOS and Android. The main fix is the iOS camera preview freezing for seconds at a time during detection; along the way this also cleans up shared-UI issues (box tracking, fps display) and an Android optimization.

Problems

• iOS: camera preview periodically froze for several seconds while detecting clovers
• Shared: detection boxes jumped between updates instead of tracking the clover smoothly
• Shared: the fps chip rendered a literal %d fps

Root cause (iOS freeze)

The CoreML model has NMS baked in (nms=True), which keeps it off the Neural Engine and runs inference on the GPU. That contended with the GPU-bound camera preview compositing, so every inference stalled the preview.

Changes

iOS — preview freeze (CameraScanner.ios.kt)

• Pin inference to the Neural Engine with computeUnits = cpuAndNeuralEngine, freeing the GPU for the preview (the core fix)
• Call onResult directly from the inference queue instead of dispatching a Kotlin lambda to the GCD main queue every frame, which had been accumulating GC roots and triggering stop-the-world pauses
• Load the Vision request off the main thread; use an atomic busy flag and cache the request list / options map to avoid per-frame allocations
• Raise the preview preset to 720p (with fallback) and the inference rate to ~10/sec
• Raise the Kotlin/Native GC target heap (build.gradle.kts)

Shared UI (ScanScreen.kt)

• Interpolate detection boxes at display refresh rate so they glide after the clover instead of jumping between inference updates
• Mark CloverDetector @stable and memoize the frame callback so state updates don't force the CameraScanner (and its UIKitView) to recompose
• Format the fps chip in Kotlin to fix the literal %d fps; drop the now-unused fps_format string resource

Android (CloverDetector.android.kt, CameraScanner.android.kt)

• Pass the camera Bitmap straight into ONNX inference, skipping the JPEG encode/decode round-trip

[coderabbitai]

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