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

effects_plugins/__init__.py

@@ -5,6 +5,7 @@ PLUGIN_DIR = Path(__file__).parent
 
 def discover_plugins():
     from engine.effects.registry import get_registry
+    from engine.effects.types import EffectPlugin
 
     registry = get_registry()
     imported = {}
@@ -22,11 +23,13 @@ def discover_plugins():
                 attr = getattr(module, attr_name)
                 if (
                     isinstance(attr, type)
-                    and hasattr(attr, "name")
-                    and hasattr(attr, "process")
+                    and issubclass(attr, EffectPlugin)
+                    and attr is not EffectPlugin
                     and attr_name.endswith("Effect")
                 ):
                     plugin = attr()
+                    if not isinstance(plugin, EffectPlugin):
+                        continue
                     registry.register(plugin)
                     imported[plugin.name] = plugin
         except Exception:

effects_plugins/fade.py

@@ -3,7 +3,7 @@ import random
 from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 
 
-class FadeEffect:
+class FadeEffect(EffectPlugin):
     name = "fade"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -54,5 +54,5 @@ class FadeEffect:
                 i += 1
         return "".join(result)
 
-    def configure(self, cfg: EffectConfig) -> None:
-        self.config = cfg
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config

effects_plugins/firehose.py

@@ -7,7 +7,7 @@ from engine.sources import FEEDS, POETRY_SOURCES
 from engine.terminal import C_DIM, G_DIM, G_LO, RST, W_GHOST
 
 
-class FirehoseEffect:
+class FirehoseEffect(EffectPlugin):
     name = "firehose"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -68,5 +68,5 @@ class FirehoseEffect:
             color = random.choice([G_LO, C_DIM, W_GHOST])
             return f"{color}{text}{RST}"
 
-    def configure(self, cfg: EffectConfig) -> None:
-        self.config = cfg
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config

effects_plugins/glitch.py

@@ -5,7 +5,7 @@ from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 from engine.terminal import C_DIM, DIM, G_DIM, G_LO, RST
 
 
-class GlitchEffect:
+class GlitchEffect(EffectPlugin):
     name = "glitch"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -33,5 +33,5 @@ class GlitchEffect:
         o = random.randint(0, w - n)
         return " " * o + f"{G_LO}{DIM}" + c * n + RST
 
-    def configure(self, cfg: EffectConfig) -> None:
-        self.config = cfg
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config

effects_plugins/noise.py

@@ -5,7 +5,7 @@ from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 from engine.terminal import C_DIM, G_DIM, G_LO, RST, W_GHOST
 
 
-class NoiseEffect:
+class NoiseEffect(EffectPlugin):
     name = "noise"
     config = EffectConfig(enabled=True, intensity=0.15)
 
@@ -32,5 +32,5 @@ class NoiseEffect:
             for _ in range(w)
         )
 
-    def configure(self, cfg: EffectConfig) -> None:
-        self.config = cfg
+    def configure(self, config: EffectConfig) -> None:
+        self.config = config