TUI freezes during operations: event loop and rendering bottlenecks

[bug-ops]

Problem

The TUI interface sometimes freezes during operations, preventing input in the text field. Root cause analysis identified two categories of issues:

1. Event loop starvation -- the tui_loop processes one event per iteration then draws a full frame. During fast streaming or parallel tool execution, agent events flood the queue while keyboard events are starved.

2. Expensive synchronous rendering -- tree-sitter syntax highlighting and markdown parsing run on every frame for all visible messages, adding latency proportional to content size.

Sub-issues

• Batch agent events and prioritize input in TUI event loop #500 Batch agent events and prioritize input in TUI event loop (P0)
• Cache syntax highlighting and markdown rendering per message #501 Cache syntax highlighting and markdown rendering per message (P1)

Analysis

Full analysis: .local/plan/tui-freeze-analysis.md

Acceptance Criteria

• Keyboard input remains responsive during fast LLM streaming
• Keyboard input remains responsive during parallel tool execution
• Frame time stays under 16ms for typical chat content