feat: braille oscilloscope / VU meter widget (TUI-014) (#284)

* feat: add braille oscilloscope / VU meter widget (TUI-014)

Renders agent activity as real-time waveforms using Unicode braille
characters at 2x4 subpixel resolution per terminal cell. Includes
BrailleCanvas with Bresenham line drawing, peak hold tracking, grid
overlay, and graceful fallbacks (sparkline, ASCII) for accessibility
and non-braille terminals.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* style: fix import sorting for ruff I001

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: chernistry