iOS: explicit line breaks expand short text to maxWidth #990

Short text separated by explicit line breaks reports the container's full available width on iOS. For example, one\ntwo in a content-sized view with a 300-point maximum width expands to 300 points even though both lines are short.

ENRMFinalizeMeasuredTextSize currently treats any second line fragment as wrapping. Explicit newlines, CRLF, and Unicode line/paragraph separators therefore trigger the same width pin as an actual soft wrap.

I propose pinning to maxWidth only when TextKit's next line starts without an explicit separator. This preserves the existing protection against rewrapping soft-wrapped content while allowing explicit breaks to retain their intrinsic width. The same helper is used by view-backed and view-free measurement.

Six native TextKit regression methods cover single-line text, explicit separators, trailing newlines, soft wrapping, soft wrapping after an explicit break, and emoji before a break. The original measurement code fails the explicit-break cases; the proposed change passes. The change was also verified in an iOS simulator through a consuming app. XCTest coverage is ready for the upstream example-app test target.

This is separate from the narrow wrapping reported in #517. The proposed fix addresses short explicit lines expanding to the full container width.

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