feat: transition effects system with multiple effects

[HerbHall]

Description

Expand the transition system beyond digital rain to support multiple visual effects. Users can select a specific effect, enable/disable transitions entirely, or use a random effect each time.

Proposed Effects

Research and prototype these candidates:

• Digital rain (current) -- Matrix-style falling characters
• Dissolve -- Characters randomly appear/disappear
• Wipe -- Left-to-right, top-to-bottom, or diagonal reveal
• Fade -- Gradual character density change
• Spiral -- Characters appear in a spiral pattern from center
• Typewriter -- Characters appear left-to-right, line by line
• Scatter -- Old screen shatters, new screen assembles
• Blinds -- Horizontal or vertical blind strips

Configuration

• transition_enabled: true|false -- master toggle
• transition_effect: "rain"|"dissolve"|"wipe"|"random" -- effect selection
• transition_speed: "fast"|"normal"|"slow" -- duration control
• transition_color: "green"|"cyan"|... -- effect color

Architecture

• Define a Transitioner interface in internal/transition/
• Each effect implements the interface
• Factory function selects effect based on config
• Random mode picks from enabled effects

Research Needed

• Which effects look best in a terminal (limited to character cells)?
• Performance: some effects need per-cell updates -- profile frame times
• What's the sweet spot for transition duration? (current digital rain feels good)

Dependencies

• Depends on feat: user configuration system with CLI flags #41 (configuration system) for effect selection
• Depends on feat: color theme system for effects and UI #44 (color theme system) for effect colors