feat: add streaming output renderer

[zgc37359-lang]

Summary

Add StreamingRenderer class that implements the stable prefix + unstable suffix pattern for incremental command output rendering. This reduces visual flicker during long-running tasks like npm test or pytest.

Changes

• mini_agent/streaming.py: New module with StreamingRenderer class

  • update(): Processes incremental content, returns stable prefix
  • render(): Returns complete rendered string
  • clear(): Resets renderer state
  • Helper functions: erase_lines(), move_cursor_up(), clear_line()

• tests/test_streaming.py: 18 test cases covering all functionality

Design

Lines ending with \n are considered "stable" and will not be re-rendered. The last line (without a trailing newline) is considered "unstable" and will be updated on each render. This pattern is borrowed from Claude Code's approach to handling streaming output.

Test plan

• All 18 tests pass
• Manual testing with long-running commands

🤖 Generated with Claude Code

[zgc37359-lang]

This PR addresses issue #71 - streaming output support for LLM responses. The StreamingRenderer class provides incremental rendering that reduces visual flicker during long-running tasks.