Optimize CharMap constructor allocations

[Copilot]

Fixes

CharMap eagerly built full Unicode row indexes during construction, making it one of the largest built-in View allocations. This changes the default path to compute row positions on demand while preserving filtered Unicode-category behavior.

Proposed Changes/Todos

• Avoid default List/Dictionary row-index allocation in CharMap
• Use arithmetic row mapping for unfiltered Unicode rows
• Keep lazy row-index collections for ShowUnicodeCategory
• Add coverage for default allocation behavior and filtered row rebuilds

if (!ShowUnicodeCategory.HasValue)
{
    _visibleRowStarts = null;
    _rowStartToVisibleIndex = null;
    SetContentSize (...);
    return;
}

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
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• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[BDisp]

LGTM. I had previously encountered an exception when navigating surrogate codepoints between 0xD800 and 0xDFFF, but this fix will prevent that from happening again.