feat: replace ref-based messages with component-based message bubbles

[avoidwork]

Summary

Replace the ref-based message array architecture in the TUI with a component-based system where each message is a standalone MessageBubble component managing its own state.

Motivation

The current architecture uses messages state array + array spreading ([...prev]) + element mutation on every render. This fights React's model and causes:

• Memory leaks from constant array allocation during streaming
• Unnecessary re-renders of unchanged messages
• Complex scroll management that doesn't work reliably
• ~12 scattered setMessages(prev => { clone + mutate }) call sites across app.js

Changes

• New: src/tui/messageBubble.js — Standalone MessageBubble component with internal state and imperative update() method via React.forwardRef
• New: src/tui/messageList.js — Manages array of MessageBubble instances with imperative addMessage()/updateMessage()/clear() API and scroll management
• Modified: src/tui/conversationPanel.js — Simplified to a thin wrapper (delegates to MessageList)
• Modified: src/tui/app.js — Replaces array-mutation state updates with imperative ref calls
• New: openspec/changes/... — Full OpenSpec proposal, design, specs, and tasks

Spec Coverage

• component-message-bubbles (new spec): 15 requirement scenarios covering MessageBubble state, MessageList imperative API, scroll management, and rendering window
• tui-scroll-view (modified spec): Delta updating scroll responsibility from ConversationPanel to MessageList

Implementation Plan

See openspec/changes/replace-ref-based-messages-with-component-message-bubbles/tasks.md for 47 implementation tasks across 8 groups.

Testing

• Unit tests for MessageBubble (rendering, update behavior)
• Unit tests for MessageList (addMessage, updateMessage, clear, setMessages)
• Update ConversationPanel tests
• Visual verification: user messages appear on send, assistant streams, interrupt clears

Security

No new dependencies, no network changes. All changes are internal TUI component architecture.