perf(editor): Flyweight TextStyle cache in syntax_highlight_isolate to eliminate hundreds of copyWith allocations #1543

Problem

In segmentsToTextSpan and _styleFromSegment (lib/core/editor/syntax_highlight_isolate.dart), (base ?? const TextStyle()).copyWith(...) is called on every single token segment, along with looping through FontWeight.values and allocating Color objects.
For a 500-token SQL query, this creates 500 distinct TextStyle instances on every syntax re-span, even though modern editor themes only contain 6-10 unique styling combinations.

Solution

• Implement a static/memoized Flyweight style cache keyed by composite style ID: (colorArgb | (fontWeight << 32) | (fontStyle << 40)).
• Reuse existing TextStyle references for identical token styles across the entire document.

Benefits

• Eliminates hundreds of TextStyle and Color allocations per highlight pass.
• Faster text span construction during interactive editing.