Scope messenger hierarchy across Core, Arc.React, and Arc.React.MVVM

[Copilot]

Summary

Implements hierarchical messenger scoping so child publishes stay local by default, while parent messages trickle recursively through descendants. Moves messenger behavior into Arc core and wires React/MVVM to resolve the nearest scoped messenger with Arc root fallback.

Added

• Added @cratis/arc/messaging (IMessenger, Message, Messenger) with parent/child hierarchy and per-scope propagation controls: bubbleToParent and trickleDownToChildren (Scoping Messenger #2231)
• Added Arc.React messaging primitives: MessengerScope, MessengerScopeContext, and useMessenger() for nearest-scope resolution (Scoping Messenger #2231)
• Added specs covering default and override propagation behavior in Core/React/MVVM touchpoints (Scoping Messenger #2231)
• Added messaging documentation pages for Core and React, and expanded MVVM messaging docs for scoped behavior (Scoping Messenger #2231)

Changed

• Arc root context now initializes and provides a root messenger instance from <Arc /> (Scoping Messenger #2231)
• withViewModel() now injects the nearest messenger into MVVM child containers, falling back to Arc root when no local scope exists (Scoping Messenger #2231)
• Package exports updated so messaging is consumable via @cratis/arc/messaging and @cratis/arc.react/messaging (Scoping Messenger #2231)

import { Arc } from '@cratis/arc.react';
import { MessengerScope, useMessenger } from '@cratis/arc.react/messaging';

const Feature = () => {
  const messenger = useMessenger();
  messenger.publish(new UserSelected('42'));
  return null;
};

<Arc>
  <MessengerScope bubbleToParent={false} trickleDownToChildren={true}>
    <Feature />
  </MessengerScope>
</Arc>;

Fixed

• Fixed propagation semantics to match issue requirements: no default upward bubbling, recursive downward forwarding by default, and explicit per-scope overrides (Scoping Messenger #2231)
• Fixed MVVM messenger binding to register a concrete core messenger instance (compatible with parent-aware constructor) (Scoping Messenger #2231)

Removed

• Removed MVVM-owned messenger implementation logic in favor of core messaging re-exports and shared runtime behavior (Scoping Messenger #2231)

Security

• No security-specific changes introduced in this PR (Scoping Messenger #2231)

Deprecated

• No deprecations introduced in this PR (Scoping Messenger #2231)

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[github-actions]

NuGet packages for this PR, e.g. Cratis.Arc:
https://github.com/cratis/arc/packages/1655206?version=20.24.25