fix: support partial text selection in <Text selectable>

[raajnadar]

Summary:

On iOS, <Text selectable> can only copy the whole paragraph. There is no highlight and no drag handles, so you cannot select part of it. Android has allowed this since the prop was added.

Fixes #13938, open since 2017. Also covers #55187, which reports the same gap as an iOS 26 regression‚ it is not a regression. iOS selection has always copied the whole block.

Approach

A selectable paragraph now renders in a non-editable UITextView. The text view is created with the same NSTextContainer that RCTTextLayoutManager already used to measure the paragraph, so the layout is unchanged and UIKit only handles the selection:

NSTextStorage *textStorage = [_selectionLayoutManager textStorageForNSAttributedString:attributedText
                                                                   paragraphAttributes:_paragraphAttributes
                                                                                  size:contentFrame.size];
NSTextContainer *textContainer = textStorage.layoutManagers.firstObject.textContainers.firstObject;
_selectableTextView = [[RCTSelectableTextView alloc] initWithFrame:contentFrame textContainer:textContainer];

The text view is also pinned so it cannot reflow: widthTracksTextView and heightTracksTextView are off, lineFragmentPadding is zero, and both insets are zero. UIKit performs the selection; React Native keeps the layout.

Behaviour change to be aware of

Copy now copies the selected range rather than the whole paragraph. That is the point of the fix, but an app relying on "long press copies everything" will see a difference.

Changelog:

[IOS] [FIXED] - <Text selectable> now supports partial text selection, with drag handles and the standard edit menu, instead of copying the whole block

Test Plan:

Tested on RNTester on an iPhone 16 Pro simulator, iOS 18.6, built with:

xcodebuild build -workspace RNTesterPods.xcworkspace -scheme RNTester -configuration Debug \
  -destination 'platform=iOS Simulator,name=iPhone 16 Pro'

Before and after were captured on the same simulator, with the same paragraph and the same gesture

	Before	After
Long press, then drag	One Copy bubble for the whole block, no highlight, no handles	A substring selected, both drag handles work

Each remaining case rendered a selectable paragraph beside an identical non-selectable control, so any difference is attributable to this change.

Case	Result
Edit menu	Copy, Look Up and Translate, acting on the selected range
onPress on a nested <Text>	Fires on a short tap; tap counter matched the control exactly
numberOfLines={2}	Same truncation point and ellipsis as the control
Inline <View> child	Same position and text flow as the control
Console	No warnings or errors

yarn format reports no changes on the three files.

[javache]

This is quite a change in behaviour. Should these be separate properties so developers can choose which one they want?