docs(qr): scanner snappiness research + companion-dev handover; 10/s native decode

Research findings and a concrete split of work: web-side items for this
repo (pre-warm camera, torch toggle, continuous focus, keep-stream-
alive) and a native handover list for the companion dev (pre-warmed
CameraX + ML Kit, QR-only format, 720p keep-latest analysis, torch,
zoom nudge, haptic dismiss) with acceptance criteria and how to
measure. Quick win landed now: live scan runs at 10 scans/sec when the
platform has a native BarcodeDetector, keeping 4/s only for the
JS-worker fallback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/src/components/WalletScanModal.vue

@@ -386,9 +386,11 @@ async function startScanning() {
         returnDetailedScanResult: true,
         highlightScanRegion: false,
         preferredCamera: 'environment',
-        // 4/s decodes plenty fast for a hand-held QR while keeping the
-        // preview smooth on phone WebViews (10/s visibly lagged the video).
-        maxScansPerSecond: 4,
+        // With a native BarcodeDetector (Chrome/Android WebView) decoding is
+        // hardware-cheap — scan at 10/s for a snappier lock-on. The 4/s cap
+        // remains for the JS-worker fallback, where 10/s visibly lagged the
+        // preview on phone WebViews.
+        maxScansPerSecond: 'BarcodeDetector' in window ? 10 : 4,
       }
     )
     await qrScanner.value.start()