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

AGENTS.md

@@ -159,6 +159,31 @@ mise run test-cov
 
 The project uses pytest with strict marker enforcement. Test configuration is in `pyproject.toml` under `[tool.pytest.ini_options]`.
 
+### Test Coverage Strategy
+
+Current coverage: 56% (336 tests)
+
+Key areas with lower coverage (acceptable for now):
+- **app.py** (8%): Main entry point - integration heavy, requires terminal
+- **scroll.py** (10%): Terminal-dependent rendering logic
+- **benchmark.py** (0%): Standalone benchmark tool, runs separately
+
+Key areas with good coverage:
+- **display/backends/null.py** (95%): Easy to test headlessly
+- **display/backends/terminal.py** (96%): Uses mocking
+- **display/backends/multi.py** (100%): Simple forwarding logic
+- **effects/performance.py** (99%): Pure Python logic
+- **eventbus.py** (96%): Simple event system
+- **effects/controller.py** (95%): Effects command handling
+
+Areas needing more tests:
+- **websocket.py** (48%): Network I/O, hard to test in CI
+- **ntfy.py** (50%): Network I/O, hard to test in CI
+- **mic.py** (61%): Audio I/O, hard to test in CI
+
+Note: Terminal-dependent modules (scroll, layers render) are harder to test in CI.
+Performance regression tests are in `tests/test_benchmark.py` with `@pytest.mark.benchmark`.
+
 ## Architecture Notes
 
 - **ntfy.py** and **mic.py** are standalone modules with zero internal dependencies
@@ -169,13 +194,15 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 
 ### Display System
 
-- **Display abstraction** (`engine/display.py`): swap display backends via the Display protocol
-  - `TerminalDisplay` - ANSI terminal output
-  - `WebSocketDisplay` - broadcasts to web clients via WebSocket
-  - `SixelDisplay` - renders to Sixel graphics (pure Python, no C dependency)
-  - `MultiDisplay` - forwards to multiple displays simultaneously
+- **Display abstraction** (`engine/display/`): swap display backends via the Display protocol
+  - `display/backends/terminal.py` - ANSI terminal output
+  - `display/backends/websocket.py` - broadcasts to web clients via WebSocket
+  - `display/backends/sixel.py` - renders to Sixel graphics (pure Python, no C dependency)
+  - `display/backends/null.py` - headless display for testing
+  - `display/backends/multi.py` - forwards to multiple displays simultaneously
+  - `display/__init__.py` - DisplayRegistry for backend discovery
 
-- **WebSocket display** (`engine/websocket_display.py`): real-time frame broadcasting to web browsers
+- **WebSocket display** (`engine/display/backends/websocket.py`): real-time frame broadcasting to web browsers
   - WebSocket server on port 8765
   - HTTP server on port 8766 (serves HTML client)
   - Client at `client/index.html` with ANSI color parsing and fullscreen support
@@ -186,6 +213,15 @@ The project uses pytest with strict marker enforcement. Test configuration is in
   - `sixel` - Sixel graphics in supported terminals (iTerm2, mintty, etc.)
   - `both` - Terminal + WebSocket simultaneously
 
+### Effect Plugin System
+
+- **EffectPlugin ABC** (`engine/effects/types.py`): abstract base class for effects
+  - All effects must inherit from EffectPlugin and implement `process()` and `configure()`
+  - Runtime discovery via `effects_plugins/__init__.py` using `issubclass()` checks
+  
+- **EffectRegistry** (`engine/effects/registry.py`): manages registered effects
+- **EffectChain** (`engine/effects/chain.py`): chains effects in pipeline order
+
 ### Command & Control
 
 - C&C uses separate ntfy topics for commands and responses