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 rounds both the container height and the measured height to the pixel grid with RCTRoundPixelValue() before comparing them. A float error far below a pixel rounds away, so the frame above fits again, while a container that is genuinely short of the text on the pixel grid (for example lineHeight: 20.25 in a 20pt frame on a 3x screen) still counts as not fitting and shrinks as before. Snapping to the pixel grid matches what the Fabric measure path already does (RCTTextLayoutManager measureAttributedString: ceils to pointScaleFactor). compareToSize: has no layout context in scope, so the scale comes from RCTScreenScale() via RCTRoundPixelValue() in RCTUtils. 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, React-Core built from source) and rebuilding the app, on an iPhone 17 simulator (3x, iOS 26.5). The number next to each row is the onLayout height.

The ten-row layout from the RNTester example. Row 2 gets a frame of 19.99999237060547 (the #52642 case, which collapsed before this change) and row 8 gets 20.000015258789062; all ten rows stay at 12pt. Below them, 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 (the width check is untouched and that frame equals the line height):

Single-line rows with lineHeight from 20.05 to 20.5 in a 20pt frame. With this change, lineHeight 20.25, 20.4 and 20.5 shrink, and 20.05 to 20.2 and 20.3 are not shrunk:

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[antoinerousseau]

got this suggestion from @coderabbitai:

Compare pixel-rounded heights, not a one-pixel range.

usedRectForTextContainer: returns the raw measured height. React Native rounds that height only later, after scaleFontSizeToFitSize: has already run. The added tolerance can therefore classify a raw height that exceeds the text container as fitting. The same storage is then drawn in the bounded text view, so the excess can remain clipped.

[nowex35]

Thanks — agreed the 1px range is looser than it needs to be. I've changed compareToSize: to round both the measured height and the container height to the pixel grid (RCTRoundPixelValue) before comparing.

What this fixes: the Yoga float error from #52642 (container 19.99999237 vs measured 20). It also fixes the reverse case, where the measured height is a ULP above the grid, and the larger frame drift Yoga produces at large y offsets (around 1e-3pt beyond y≈2^15).

Trade-off: overflow of less than half a pixel is treated as fitting (the previous revision allowed up to 1px). In my tests, overflow of up to 0.75px stayed inside the line's bottom padding and wasn't visibly clipped.

Compared with main: I found no case where it shrinks text that main renders at full size. Ceil-both rounding gave position-dependent results, and a fixed epsilon failed at large y.

Tested on iPhone 17 (3x); screenshots updated in the description.