perf(editor): eliminate syntax highlight flicker and fallback to plain text on documents >= 8KB in QueryaHighlightController #1351

Summary

In lib/core/editor/querya_highlight_controller.dart (lines 40–82):
When editing documents where text.length >= kSyntaxHighlightIsolateThreshold (8192 characters / ~200 lines of SQL):

1. On each keystroke, buildTextSpan checks:
   if (_cachedSpan != null && _cachedText == text)
2. Because the user just typed a character, text changed and _cachedText == text is false.
3. The method falls through to:
   return TextSpan(style: style, text: text);
   which returns completely unhighlighted, plain monochrome text.
4. Then _debounceTimer waits 100 ms (kSyntaxHighlightDebounce), launches the isolate highlight job, and once completed, updates _cachedSpan and calls notifyListeners().

As a result, while typing in any script >= 8 KB, the entire editor visibly flashes to monochrome plain text on every keystroke, and only flashes back to highlighted colors 100+ ms after the user pauses typing. If the user types continuously, all syntax highlighting is completely lost during typing.

Proposed Solution

1. Preserve _cachedSpan during active typing while background isolate highlighting is in flight, rather than falling back to plain monochrome text.
2. Provide a lightweight local line-level patch or keep the stale highlight span until the new isolate result arrives.
3. Only update the active span once the worker isolate returns the new segments.

Acceptance Criteria

• Typing in scripts >= 8 KB does not drop syntax highlighting or cause monochrome text flickering.
• Syntax highlighting updates seamlessly once isolate worker returns updated spans.
• Frame timing during typing remains within 60/120 Hz budget without jank.