iOS: add zoomCoverScale so the flying cover matches a zoomed card

[artemlitch]

Problem

Bookwise scales its book covers by 1.01 inside a clipping box, to hide a keyline along the image edge. The zoom flight does not know that.

zoomMakeStandInFromCardView rasterises the card into a flat stand-in, so the flight shows the cover at 1.0 while the card behind it sits at 1.01. The cover visibly changes size at the moment the flight hands over to the real card.

This is currently broken in production. readwiseio/rekindled already passes zoomCoverScale from LoggedInRootContainer.tsx, but nothing downstream accepts it, so the value is dropped and the prop is a no-op.

Fix

zoomCoverScale is a per-screen scale applied to the rasterised contents about the canvas centre, inside zoomMakeStandInFromCardView.

static inline CGFloat RNSZoomCoverScale(CGFloat override)
{
  return override > 0 ? override : 1;
}

Non-positive means no zoom, so every existing screen is unchanged: RNSZoomCoverScale maps the 0 default to 1, and the CGContextScaleCTM block is skipped entirely when the value is 1.

Plumbed exactly like the other Readwise zoom props (zoomCloseOvershoot and its siblings): native prop on RNSScreen, Fabric spec entries for both the screen and modal-screen components, ScreenProps, the InnerScreen forwarder, and NativeStackNavigationOptions.

iOS only. 45 lines added, none removed.

Base

Branched from readwise, not from #5. The stand-in machinery it hooks into is already on readwise, so this applies cleanly and can merge independently of the modal-zoom work.

Consumer side

Landing this is necessary but not sufficient. The #readwise ref has to be bumped, and patches/@react-navigation__native-stack@7.12.0.patch in rekindled needs zoomCoverScale added to its prop allow-list, or the navigator drops the prop before it reaches a screen. Being a native prop, it also needs a native build — it cannot ship over OTA.