D-14: Message views — UserMessageView + AssistantMessageView with live caret

[kirich1409]

Description

Implement the two message views for Dialogue — user bubble (right side) and assistant bubble (left side, with streaming caret, fade-in, thinking block, tool call markers).

Spec: Epic #250 §6 (UI concept — message level live feel).

Scope

Files

• UserMessageView.swift — plain right-aligned bubble.
• AssistantMessageView.swift — left-aligned, contains markdown rendering + tool call markers + thinking block.
• LiveCaret.swift — reusable blinking cursor ▊ component.

UserMessageView

```swift
public struct UserMessageView: View {
public let message: AgentMessage
}
```

• Renders message.content as plain text (user input is not markdown-formatted in MVP).
• Right-aligned with HStack { Spacer() }.
• Card(style: .plain) wrapper from DS.
• Max width 80% of available — otherwise balloons with 1-char messages.

AssistantMessageView

```swift
public struct AssistantMessageView: View {
public let message: AgentMessage
public let toolCalls: IdentifiedArrayOf // to look up by ID
public let isStreaming: Bool
}
```

• Renders message.content[] segments in order:
  • .text(String) → DialogueMarkdownView(source: text, streaming: isStreaming, cache: shared).
  • .thinking(Thinking) → ThinkingIndicatorView (D-17).
  • .toolCall(ToolCallID) → lookup in toolCalls, render via ToolCallCardView (D-15/D-16).
  • .citation → inline footnote marker.
• Live caret ▊ at end of last .text segment while isStreaming == true.
• Fade-in animation on message appear (.transition(.opacity.combined(with: .move(edge: .bottom))), DS.Motion.fast).
• Active-streaming glow pulse (1–2% opacity tint overlay) — guarded by accessibilityReduceMotion.

LiveCaret

Small ▊ character with .opacity animation (0.3 ↔ 1.0, 0.5s cycle). Disabled under reduceMotion (show static).

Accessibility

• .accessibilityLabel on each message: "Claude's message" / "Your message".
• .accessibilityValue includes content (markdown stripped to plain text for VoiceOver).
• Live region announce when new assistant message completes (message_stop).
• Thinking block and tool cards — own accessibility; not announced separately here.

Acceptance Criteria

• UserMessageView + AssistantMessageView implemented with public init.
• Live caret ▊ shown during streaming, hidden when isStreaming = false.
• Fade-in works on appear; respects reduceMotion (no slide-in under reduceMotion).
• Multi-segment assistant message (text → tool_call → text → thinking) renders in correct order.
• .textSelection(.enabled) allows copying either message content.
• A11y: VoiceOver reads messages in reading order; only new message-complete announces (not per-token).
• Snapshot tests: user message, short assistant, long assistant with mixed content, all in 4 appearances.
• No hardcoded values — DS tokens only.

Relationships

• Parent issue: EPIC: Dialogue — structured chat UI for agent sessions #250
• Blocked by: #D-13 (markdown), #D-15 (ToolCallCardView lookup), #D-17 (ThinkingIndicatorView), D-1: DesignSystem — add Card primitive #251 (Card), D-2: DesignSystem — add StatusIndicator primitive #252 (StatusIndicator)
• Blocks: #D-18 (DialogueView)