feat(code-editor): caret position tick + selection range block in CodeScrollMarkerPanel

[abbaye]

Summary

Extend CodeScrollMarkerPanel with two new visual indicators overlaid on the vertical scrollbar:

• A bright yellow tick for the current caret line.
• A semi-transparent blue block spanning the active multi-line selection.

This gives VS-like at-a-glance position awareness on long files without any additional UI chrome.

Implementation

CodeScrollMarkerPanel

• New state fields: _caretLine, _selectionStart, _selectionEnd.
• New API: UpdateCaretAndSelection(int caretLine, int selStart, int selEnd, int totalLines).
  • selStart == -1 → no selection block rendered.
  • totalLines parameter used as proportional denominator (fold-corrected).
• Z-order in OnRender: selection block → word-highlight ticks → caret tick (caret always topmost).
• Static frozen brushes:
  • s_caretTick — ARGB(220, 255, 255, 100) bright yellow.
  • s_selectionBlock — ARGB(70, 86, 156, 214) semi-transparent blue.

CodeEditor wiring

• Called from the render pass (OnRender) every frame with the current _cursorLine, selection bounds (only when selection spans multiple lines), and fold-corrected visibleLines.

Affected files

• Sources/WpfHexEditor.Editor.CodeEditor/Controls/CodeScrollMarkerPanel.cs
• Sources/WpfHexEditor.Editor.CodeEditor/Controls/CodeEditor.cs — OnRender