feat(TextBlock): TextTrimming

[ramezgerges]

GitHub Issue: closes https://github.com/unoplatform/kahua-private/issues/406, closes #18716

The branch is on top of #22448

[Copilot]

Pull request overview

This PR implements TextTrimming support (CharacterEllipsis and WordEllipsis) for TextBlock controls in Skia-based platforms, along with spell checking capabilities and text highlighting features. The implementation builds on PR #22448 for text highlighters and introduces significant infrastructure for font fallback and spell checking.

Changes:

• Added TextTrimming support with character-level and word-level ellipsis algorithms
• Implemented spell checking service using Hunspell dictionaries with visual red underlines
• Added TextHighlighter API for marking text ranges with custom foreground/background colors
• Introduced font fallback service infrastructure for better Unicode character support

Reviewed changes

Copilot reviewed 133 out of 138 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
UnicodeText.skia.cs	Core text layout engine with trimming, spell checking, and font fallback logic
TextBlock.skia.cs	Integration of TextHighlighters collection and spell checking support
TextBox.skia.cs	Spell checking UI with context menu suggestions
TextHighlighter.cs	New API for text highlighting with customizable brushes
TextRange.cs	Structure representing text range with start index and length
ISpellCheckingService.cs	Interface for spell checking implementations
IFontFallbackService.skia.cs	Interface for font fallback providers
FontFallbackService.skia.cs	Noto fonts-based fallback implementation
SpellCheckingService.cs	Hunspell-based spell checking implementation
FontFallbackPreprocessor	Tool to generate font coverage maps from Noto fonts
Uno.WinUI.SpellChecking.csproj	New add-in package for spell checking
Resource files	Localized strings for spell checking UI (80+ languages)

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Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

src/Uno.UI/UI/Xaml/Documents/UnicodeText.skia.cs:786

• Typo: lineBreakingOpporunities / lineBreakingOpporunity should be lineBreakingOpportunities / lineBreakingOpportunity for readability and consistency.

	private static List<List<BidiRun>> ApplyLineBreaking(float lineWidth, List<BidiRun> logicallyOrderedRuns,
		List<(int endRunIndex, int endIndexInLastRun)> lineBreakingOpporunities, bool rtl,
		TextWrapping textWrapping)
	{
		var lines = new List<List<BidiRun>>();

		(int firstRunIndex, int startingIndexInFirstRun) = (0, 0);
		(int endRunIndex, int endIndexInLastRun)? prevInSameLine = null;

		for (var lineBreakingOpportunityIndex = 0; lineBreakingOpportunityIndex < lineBreakingOpporunities.Count; lineBreakingOpportunityIndex++)
		{
			var lineBreakingOpporunity = lineBreakingOpporunities[lineBreakingOpportunityIndex];

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