feat: CRT phosphor shader engine and retro themes (#281)

* feat: add CRT phosphor shader engine and retro themes

Implements CRTShader class that transforms Rich Text output to simulate
a 1987 CRT monitor with scanline dimming, phosphor bloom, chromatic
aberration at screen edges, and monochrome palette mapping (amber, green,
cool white). Adds three retro CRT themes to the theme system.

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

* style: fix ruff lint warnings in crt_shader.py

Remove unused noqa directives and simplify if-else to ternary.

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

* fix: resolve pyright strict-mode errors in crt_shader.py

Use get_style_at_offset() public API instead of accessing private _spans.
Add Console import for style resolution. Suppress reportUnusedFunction
for _to_monochrome which is part of the public helper API.

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

* style: ruff format crt_shader.py

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