Add a blinking text caret

[andrewtheart]

What

Adds a blinking text caret. Currently the caret is drawn solid on every cursor-canvas redraw and never blinks.

How

A small CaretBlinkManager drives the blink while the control is focused:

• Toggles an IsCaretVisible flag on a DispatcherQueueTimer at 530 ms (the Windows default caret interval).
• CursorRenderer.Draw gates only the RenderCursor(...) call on IsCaretVisible. The current-line highlighter stays unconditional, so the highlighted line doesn't flicker while the caret blinks. Because Win2D clears the caret canvas each draw, skipping the paint yields a real blink.
• Cursor activity keeps the caret solid so it never blinks mid-keystroke: CanvasUpdateManager.UpdateCursor() calls NotifyCaretActivity() (resets the phase). The blink timer redraws via a separate RedrawCursorForBlink() that does not reset the phase (otherwise the caret would never toggle).
• The timer runs only while focused: FocusManager.SetFocus() → Start(), RemoveFocus() → Stop(), and Unload() stops it too.

Files

• New Core/CaretBlinkManager.cs.
• Core/CoreTextControlBox.xaml.cs — construct/init the manager, pass it to CursorRenderer.Init, stop it in Unload.
• Core/Renderer/CursorRenderer.cs — gate the caret paint on IsCaretVisible.
• Core/FocusManager.cs — start/stop on focus change.
• Core/CanvasUpdateManager.cs — NotifyCaretActivity() + RedrawCursorForBlink().

Testing

• Builds clean (TextControlBox.csproj, Release/x64).
• Focused: caret blinks at ~530 ms; typing/moving keeps it solid for a full interval then resumes. Unfocused: caret is not painted (unchanged). No blink timer runs while unfocused.

Contributing this back upstream after adding it for a downstream editor built on TextControlBox.