feat(core): expose resolved accessibility attributes on ComponentNode instead of props['@accessibility'] #4166

Summary

Expose resolved AccessibilityAttributes (label, description, live, hidden) as a dedicated first-class reactive property on ComponentNode (analogous to node.metadata added in #3469 / #3708), rather than storing and resolving @accessibility (and mirroring legacy accessibility) inside node.props.

Background & Problem

With the v1.0 @-prefixed component envelope cutover (#4037, #4044), @accessibility is a component envelope mixin alongside @metadata, @catalogId, and @checks, leaving the bare name accessibility free for custom catalog component properties.

Currently, unlike @metadata (which is extracted from ComponentModel.properties and exposed on ComponentNode.metadata), @accessibility remains inside ComponentModel.properties and is resolved by GenericBinder / NodeResolver into node.props['@accessibility'] (plus a conditional mirror onto node.props['accessibility'] when the component schema does not declare a custom accessibility property).

This creates several ergonomic and architectural warts across Core SDKs and framework adapters:

1. Manual probing & unwrapping in every adapter: Framework adapters and universal helpers must probe props['@accessibility'] ?? props['accessibility'], verify it is an attributes object rather than a custom component property, and manually unwrap or subscribe to each nested ResolvedBinding (label, description, live, hidden):
   • TypeScript (typescript/core/web_core/src/universal/host_accessibility.ts): resolveAccessibilityProps(props) loops over ['@accessibility', 'accessibility'] and unwraps bindings.
   • Flutter (dart/a2ui_flutter/lib/src/node_view.dart): reads props.value('@accessibility') ?? props.value('accessibility') and manually manages _accessibilityUnsubscribes.
   • Swift (swift/core/Sources/A2UICore/Resolution/NodeResolver.swift & swift/swiftui/Sources/A2UISwiftUI/ComponentNodeView.swift): reads @accessibility / accessibility from node.properties.
2. Envelope leakage into node.props: Component implementations inspecting node.props see @accessibility (and the conditional accessibility mirror) mixed in with their own schema-defined component properties, requiring special-case mirroring logic in GenericBinder (finalizeResolvedProps, mirrorsAccessibility) and NodeResolver.

Proposed Solution

1. ComponentNode.accessibility across Core SDKs (@a2ui/web_core, a2ui_core in Dart/Python, A2UICore in Swift):
   • Expose a dedicated reactive accessibility property on ComponentNode (e.g., readonly accessibility: Signal<ResolvedAccessibilityAttributes | undefined> in TypeScript, ReadonlySignal<ResolvedAccessibility?> in Dart, and corresponding properties on Python ComponentNode and Swift Node) with label, description, live, and hidden already resolved/unwrapped.
   • Resolve envelope @accessibility (and legacy bare accessibility on pre-1.0 or during the Remove legacy bare-key fallback for v1.0 component envelope fields #4082 transition when undeclared as a custom property) into node.accessibility instead of polluting node.props['@accessibility'].
2. Framework Adapter Simplification:
   • Update @a2ui/web_core/universal (A2uiLitElement), @a2ui/react, @a2ui/angular, a2ui_flutter (NodeView), and A2UISwiftUI (ComponentNodeView) to read accessibility state directly from node.accessibility.
3. Blueprints & Conformance:
   • Document ComponentNode.accessibility in blueprints/modules/a2ui_core.blueprint.md, blueprints/features/node_resolution.blueprint.md, and blueprints/modules/a2ui_framework_adapter.blueprint.md (alongside [Blueprints] Document AccessibilityAttributes resolution and adapter mapping in a2ui_core and a2ui_framework_adapter blueprints #3148).