perf(editor): eliminate root TextSpan reallocation on cursor blink in QueryaHighlightController #1522

Summary

In lib/core/editor/querya_highlight_controller.dart (lines 70–75):

if (_cachedText == text &&
    _cachedBrightness == brightness &&
    _cachedSpan != null) {
  return TextSpan(style: style, children: [_cachedSpan!]);
}

Whenever the text cursor blinks in the editor (every 500 ms while idle), Flutter invokes buildTextSpan(). Even when neither the text, the style, nor the syntax highlighting spans have changed, a new wrapper TextSpan instance is allocated on every blink.

Proposed Solution

1. Cache the root wrapper span:

   TextSpan? _cachedRootSpan;
   TextStyle? _cachedStyle;

2. In buildTextSpan(), return _cachedRootSpan! if text == _cachedText, style == _cachedStyle, and brightness == _cachedBrightness.
3. Invalidate _cachedRootSpan when text, style, or theme changes.

Acceptance Criteria

• Cursor blinking in idle editor state produces zero heap allocations.
• No regressions in text rendering or cursor movement.