Trim NSAttributedString construction for text on iOS

[hryhoriiK97]

Summary:

RCTNSAttributedStringFromAttributedString (ReactCommon/react/renderer/textlayoutmanager/platform/ios/.../RCTAttributedTextUtils.mm) converts a paragraph's AttributedString into the NSAttributedString TextKit lays out and draws. It runs once per paragraph per measurement when the attributed-string cache misses, so for a screen of distinct labels it runs once per label.

Current behavior. Per fragment, the conversion:

• iterated getFragments() with for (auto fragment : ...), copying each Fragment: its std::string, its TextAttributes, and a ShadowView with three shared_ptr members (props, event emitter, state), so three atomic retain/release pairs per fragment;
• built the fragment's attributes, created the string, then added the event emitter in a second addAttributes:range: pass, which splits and re-merges the attribute runs TextKit just created;
• allocated an NSMutableParagraphStyle before knowing whether any paragraph attribute was set, discarding it for the common unaligned, default-line-height case;
• computed the effective font size multiplier twice when lineHeight is set, which with dynamicTypeRamp means two UIFontMetrics round trips;
• always created an enclosing NSMutableAttributedString and appended each fragment to it, even for the single-fragment paragraph that most labels are;
• built an NSString for the font family even when it is unset and resolves to the default family anyway.

With this change. Fragments are iterated by reference; the event emitter goes into the initial attribute dictionary; the paragraph style is created only when one of its fields is set; the multiplier is computed once and passed to the font resolution; a single-fragment paragraph returns the fragment's string directly; the family string is skipped when unset. The attributes attached to each run are the same as before, in the same ranges.

Changelog:

[IOS] [CHANGED] - Build a paragraph's NSAttributedString with fewer copies and allocations per text fragment

Test Plan:

• node scripts/clang-format.js --check on the changed file passes.
• RNTester built with xcodebuild for the iPhone 16 Pro simulator (Debug) and for a physical iPhone 15 Pro (Release); the Text examples render as before.
• Screenshots of a 1000-label list and of a set of edge cases (right-aligned text with an inline view, centered multi-line text, empty text with an explicit size, truncation across nested colours, RTL with an inline view, clipped fixed height, two-line tail ellipsis) are byte-identical before and after.
• Measurement. RNTester Playground mounting 1000 unique single-line and wrapping labels in one commit; a timer around the conversion call inside -[RCTTextLayoutManager measureAttributedString:...], summed per transaction. iPhone 16 Pro simulator, Debug, medians of 3 to 5 runs:

1000 labels	before	this PR	change
conversion, system font	13.3 ms	10.7 ms	−20%
conversion, fontFamily: 'Avenir'	12.2 ms	10.9 ms	−11%

Conversion is about a quarter of the paragraph's measurement time, so this is roughly 5% of the shadow-thread layout cost of a text-heavy commit. TextKit layout itself (the remaining three quarters) is unchanged, as expected.

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