feat(tests): improve coverage to 56%, add benchmark regression tests

- Add EffectPlugin ABC with @abstractmethod decorators for interface enforcement
- Add runtime interface checking in discover_plugins() with issubclass()
- Add EffectContext factory with sensible defaults
- Standardize Display __init__ (remove redundant init in TerminalDisplay)
- Document effect behavior when ticker_height=0
- Evaluate legacy effects: document coexistence, no deprecation needed
- Research plugin patterns (VST, Python entry points)
- Fix pysixel dependency (removed broken dependency)

Test coverage improvements:
- Add DisplayRegistry tests
- Add MultiDisplay tests
- Add SixelDisplay tests
- Add controller._get_display tests
- Add effects controller command handling tests
- Add benchmark regression tests (@pytest.mark.benchmark)
- Add pytest marker for benchmark tests in pyproject.toml

Documentation updates:
- Update AGENTS.md with 56% coverage stats and effect plugin docs
- Update README.md with Sixel display mode and benchmark commands
- Add new modules to architecture section

engine/effects/legacy.py

@@ -1,6 +1,14 @@
 """
 Visual effects: noise, glitch, fade, ANSI-aware truncation, firehose, headline pool.
 Depends on: config, terminal, sources.
+
+These are low-level functional implementations of visual effects. They are used
+internally by the EffectPlugin system (effects_plugins/*.py) and also directly
+by layers.py and scroll.py for rendering.
+
+The plugin system provides a higher-level OOP interface with configuration
+support, while these legacy functions provide direct functional access.
+Both systems coexist - there are no current plans to deprecate the legacy functions.
 """
 
 import random