Editor rendering: add per-line visual cache with document-change invalidation

[Copilot]

Editor.BuildVisualLine was rebuilding visual lines for every visible row on every frame, driving high allocation churn during navigation/scroll. This change adds a per-line cache and selective invalidation so unchanged lines are reused and only affected lines are rebuilt.

• What this PR changes

  • Introduces a visual-line cache in Editor keyed by DocumentLine.
  • Reuses cached CellVisualLine instances on cache-eligible build paths (no selection/styled segments/transformers).
  • Invalidates cache entries from the first affected line onward on Document.Changed (covers insert/remove/replace and downstream line-number shifts).
  • Clears cache when rendering-affecting editor options/document wiring change (Document, SyntaxHighlighter, SyntaxLanguage, IndentationSize, ShowTabs).

• Safety/consistency details

  • Cache entries store source line number metadata and validate line identity before reuse.
  • End-of-document change offsets are handled safely during invalidation (no stale/invalid DocumentLine access).

• Tests added

  • Cache reuse for unchanged lines.
  • Invalidation of changed line and following lines after edits.

if (canUseCache
    && _visualLineCache.TryGetValue (line, out CachedVisualLine? cached)
    && cached.LineNumber == line.LineNumber
    && cached.NormalAttribute == resolvedNormal
    && cached.SelectedAttribute == resolvedSelected)
{
    return cached.Line;
}

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  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/.nuget/packages/jetbrains.resharper.globaltools/2025.2.4/tools/netcoreapp3.1/any/cleanupcode.unix.runtimeconfig.json /home/REDACTED/.nuget/packages/jetbrains.resharper.globaltools/2025.2.4/tools/netcoreapp3.1/any/cleanupcode.exe Terminal.Gui.Text.slnx --profile=Full Cleanup (dns block)
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