Improve discoverability of CommandNotBound bubbling pattern for mouse events

[tig]

Problem

When implementing mouse-wheel scrolling on a child view that should forward to a parent (e.g., a gutter subview inside an Editor's Padding that needs to scroll the Editor), the correct pattern is:

1. Child adds only MouseBindings (e.g., MouseBindings.Add(MouseFlags.WheeledUp, Command.ScrollUp)) — no AddCommand handler.
2. Parent sets CommandsToBubbleUp = [Command.ScrollUp, Command.ScrollDown, ...]
3. When the child receives the wheel event, the mouse binding fires the command, DefaultCommandNotBoundHandler runs → TryBubbleUp finds the ancestor with the command in CommandsToBubbleUp → invokes it.

This is elegant and idiomatic, but extremely hard to discover:

• command-diagrams.md focuses on Accept/Activate flows and doesn't mention scroll or mouse-wheel forwarding.
• The CommandNotBound event XML docs describe the mechanism but don't explain when to use it (i.e., "add a MouseBinding without a handler to let it bubble").
• CommandsToBubbleUp docs don't mention the AdornmentView special case (adornment.Adornment?.Parent) which is critical for views hosted in Padding/Border.
• There's no example showing the "child forwards wheel to parent" pattern which is a very common need.

Suggested Improvements

1. Add a "Mouse Event Forwarding" section to command-diagrams.md (or a new mouse-commands.md) showing:

   • The pattern: MouseBinding without AddCommand → CommandNotBound → TryBubbleUp → parent handles
   • A concrete example (e.g., ScrollBar/Gutter wheel forwarding)
   • The AdornmentView special case in CommandWillBubbleToAncestor

2. Improve CommandsToBubbleUp API docs to:

   • Explicitly mention that views in Padding/Border/Margin bubble to the Adornment's Parent (not SuperView)
   • Show a code snippet of the wheel-forwarding pattern

3. Improve CommandNotBound event docs to:

   • Clarify that adding a MouseBinding without an AddCommand handler is intentional and idiomatic
   • Link to the bubbling docs

4. Add a scenario/example in UICatalog demonstrating mouse-wheel forwarding between views.

Context

Discovered while implementing gutter mousewheel scrolling for the Editor library (gui-cs/Editor#96). The pattern works perfectly once found, but required reading View.Command.cs source and CommandBridge.cs to piece together.