Fixes #836. Improve discoverability of CommandNotBound bubbling pattern for mouse events

[Copilot]

Fixes

The mouse-wheel forwarding pattern (MouseBinding without AddCommand → CommandNotBound → TryBubbleUp → ancestor handles) works well but is undocumented and requires reading source to discover.

Proposed Changes/Todos

• Add "Mouse Event Forwarding via Command Bubbling" section to docfx/docs/mouse.md — pattern explanation, code example, AdornmentView special case
• Add Level 4 mermaid diagram to docfx/docs/command-diagrams.md showing the full flow
• Improve CommandsToBubbleUp XML docs — mouse-wheel pattern example, clarify Padding/Border/Margin bubble to Adornment.Parent
• Improve CommandNotBound/OnCommandNotBound XML docs — explain "binding without handler" is intentional and idiomatic

The documented pattern:

// Parent: handles commands and opts into bubbling
editor.CommandsToBubbleUp = [Command.ScrollUp, Command.ScrollDown];

// Child: binds mouse wheel WITHOUT AddCommand — command bubbles to ancestor
gutter.MouseBindings.Add (MouseFlags.WheeledUp, Command.ScrollUp);
gutter.MouseBindings.Add (MouseFlags.WheeledDown, Command.ScrollDown);

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