Tolerate sub-pixel height shortfall in adjustsFontSizeToFit on iOS

[nowex35]

Summary:

<Text adjustsFontSizeToFit numberOfLines={1} style={{lineHeight: N}}> sometimes collapses to the minimum font size on iOS even though the text fits, and which row it happens to depends on the row's position on screen (#52642).

Two things combine:

1. Yoga rounds the top and bottom edges of a node to the pixel grid independently and stores their difference as the height. Each rounded edge is divided by pointScaleFactor and narrowed to float, so on a 3x screen a node far enough down the screen can get a height one float ULP short of its lineHeight (19.99999237060547 for lineHeight: 20; 10.999969482421875 for 11 on a production screen). Subtract rounded edges in pixel space when rounding layout results yoga#2011 addresses that side.
2. compareToSize: in NSTextStorage+FontScaling.m requires size.height >= measuredSize.height with no tolerance. The text is laid out at the fixed line height at every font ratio, so such a frame never fits, lastRatioWhichFits keeps its initial value and the text ends at minimumFontSize (4pt before Fix minimumFontScale with adjustsFontSizeToFit in the New Architecture #58492).

This PR treats a height shortfall smaller than one physical pixel as fitting. A sub-pixel difference cannot change what is drawn, and one physical pixel is the granularity the Fabric measure path already snaps to (RCTTextLayoutManager measureAttributedString: ceils to pointScaleFactor). compareToSize: has no layout context in scope, so the scale comes from RCTScreenScale(), the same helper RCTCeilPixelValue() in RCTUtils uses. The width check is unchanged. The file is shared by Paper and Fabric, so both renderers get the same behavior.

It also adds an RNTester example (Font Size Adjustment with lineHeight) that reproduces the condition: ten lineHeight: 20 rows after a 0.5pt separator. On a 3x device one of the rows (which one depends on the device) gets a frame of 19.99999237060547 and collapsed before this change.

Fixes #52642

Changelog:

[IOS] [FIXED] - Fix adjustsFontSizeToFit text collapsing to the minimum size when Yoga's pixel-grid rounding leaves the frame a sub-pixel short of lineHeight

Test Plan:

Verified by applying the NSTextStorage+FontScaling.m change to React Native 0.86.2 (Expo SDK 57, New Architecture) and rebuilding the app, with the same ten-row layout as the RNTester example, on an iPhone 13 simulator (3x, iOS 26.5). The number next to each row is the onLayout height.

Before (row 3 has a frame of 19.99999237060547 and collapses):

After (same layout, same frame heights, no row collapses):

Regression check in the same build: a row whose text is genuinely too wide for 200pt still shrinks to fit, and a row with lineHeight: 10, fontSize: 15 is still left unscaled (both unchanged from before, since the width check is untouched and that frame equals the line height):

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