[ZhuchkaTriplesix]

Closes #1543.

_styleFromSegment ran (base ?? const TextStyle()).copyWith(...) for every segment, and segmentsToTextSpan runs on every keystroke (patched segments while typing, and after each highlight pass). A theme has a handful of token looks, a document hundreds of tokens.

• Styles are cached by (colour, has-colour, weight, font style), packed into one int, for the current base style. A new base (theme, font size) clears the cache; so does reaching 256 looks, so a pathological document cannot grow it.
• Segments with the same look now share one TextStyle instance; segments with no colour, weight or style still get null.

Not measured: no editor scenario in the bench isolates span construction. The saving is the copyWith, Color and weight lookup per token, minus a hash lookup. Tests: shared instance for equal looks, base style and attributes preserved, a different base is not served stale styles, 600 distinct looks stay correct past the cache cap.