[ZhuchkaTriplesix]

Closes #1351

Problem

In QueryaHighlightController, when editing documents where text.length >= kSyntaxHighlightIsolateThreshold (8192 characters / ~200 lines):

1. On each keystroke, buildTextSpan previously saw _cachedText == text as false and fell back to plain monochrome text (TextSpan(style: style, text: text)).
2. Only after kSyntaxHighlightDebounce (100 ms) did the worker isolate complete and call notifyListeners().
3. As a result, typing in any script >= 8KB continuously flashed the editor between unhighlighted monochrome and highlighted text.

Solution

• Introduced patchHighlightSegments in syntax_highlight_isolate.dart, which patches cached segments across untouched prefix and suffix ranges while inserting the edited region with zero latency.
• Preserved syntax colors during active typing in documents >= 8KB in QueryaHighlightController instead of dropping to monochrome plain text.
• Once the background isolate completes, segments and spans are seamlessly updated with exact grammatical tokenization without flicker or frame drops.
• Added unit tests for patchHighlightSegments and verified that syntax styling is retained during live typing in large documents.