feat: add SwiftUI rendering support

[BBC6BAE9]

Description

Native SwiftUI renderer for the A2UI protocol, enabling all Apple platforms to render agent-generated UI from A2UI JSON payloads into native SwiftUI views.

Resolves #680.

iOS	iPadOS	macOS	visionOS	watchOS	tvOS

• Platforms: iOS 17+, macOS 14+, watchOS 10+, visionOS 1+、tvOS 17+
• Spec compatibility: Targeting v0.8, since all current Agent demos and web-core implementations are based on v0.8. Support for v0.9/v0.10 will follow as the project evolves.
• All 18 standard components: Text, Image, Icon, Video, AudioPlayer, Row, Column, List, Card, Tabs, Divider, Modal, Button, TextField, CheckBox, DateTimeInput, ChoicePicker, Slider
• All catalog functions: formatString, formatNumber, formatCurrency, formatDate, pluralize, openUrl, required, email, regex, length, numeric, and, or, not
• 84 unit tests across 5 test files, 0 failures — core logic line coverage 74%
• A2A Client with SSE streaming and agent card discovery
• Custom component support via CustomComponentRegistry

Design area	Approach
State management	@Observable with per-key ObservableValue for fine-grained SwiftUI updates
List rendering	LazyVStack/LazyHStack for scroll performance
Streaming	AsyncThrowingStream-based JSONL parser
Styling	Customizable via SwiftUI Environment (A2UIStyle)
Accessibility	AccessibilityModifier applies labels/hints from component metadata

Test coverage (core logic modules)

Module                         Lines    Cover
SurfaceViewModel.swift           830    74.0%
CatalogFunctionEvaluator.swift   310    74.8%
Primitives.swift                 133    97.0%
JSONLStreamParser.swift          144    98.6%
ChecksEvaluator.swift            164    66.5%
SurfaceManager.swift              54    79.6%
AnyCodable.swift                  73    86.3%

View layer (SwiftUI components) is not unit-testable without a host app — covered by the demo app's 11 pages.

renderers/swift/              # Swift package (A2UI)
├── Sources/A2UI/             # Renderer library
├── Tests/A2UITests/          # 84 tests + 12 JSON fixtures
│   ├── MessageDecodingTests.swift      # Decoding, component parsing, JSONL, SurfaceManager
│   ├── DataBindingTests.swift          # Path resolution, templates, actions, incremental updates
│   ├── CatalogFunctionTests.swift      # Catalog functions, interpolation, date parsing
│   ├── ValidationTests.swift           # Checks: required, email, length, and/or/not
│   ├── PrimitivesTests.swift           # Codable round-trip for all value types
│   └── TestData/                       # 12 JSON fixtures
└── README.md
samples/client/swift/         # Demo app (11 pages incl. live A2A agent)
└── A2UIDemoApp/

All files are new additions. No existing files on google:main were modified.

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.

If you need help, consider asking for advice on the discussion board.

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive and well-architected SwiftUI renderer for A2UI. The use of modern SwiftUI features like @Observable and the Layout protocol is excellent, and the extensive test suite provides great coverage. I've identified a few critical issues related to error handling where errors are silently ignored, which could make debugging difficult. I've also suggested refactoring a particularly complex function to improve its maintainability and efficiency. Addressing these points will significantly enhance the robustness of the new renderer. Additionally, please note that the PR description template should be filled out as per the repository's contributing guidelines.