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

README.md

@@ -75,6 +75,7 @@ Mainline supports multiple display backends:
 
 - **Terminal** (`--display terminal`): ANSI terminal output (default)
 - **WebSocket** (`--display websocket`): Stream to web browser clients
+- **Sixel** (`--display sixel`): Sixel graphics in supported terminals (iTerm2, mintty)
 - **Both** (`--display both`): Terminal + WebSocket simultaneously
 
 WebSocket mode serves a web client at http://localhost:8766 with ANSI color support and fullscreen mode.
@@ -131,10 +132,17 @@ engine/
   translate.py          Google Translate wrapper + region detection
   render.py             OTF → half-block pipeline (SSAA, gradient)
   effects/              plugin architecture for visual effects
-    controller.py       handles /effects commands
-    chain.py            effect pipeline chaining
+    types.py            EffectPlugin ABC, EffectConfig, EffectContext
     registry.py         effect registration and lookup
+    chain.py            effect pipeline chaining
+    controller.py       handles /effects commands
     performance.py      performance monitoring
+    legacy.py           legacy functional effects
+  effects_plugins/      effect plugin implementations
+    noise.py            noise effect
+    fade.py             fade effect
+    glitch.py           glitch effect
+    firehose.py         firehose effect
   fetch.py              RSS/Gutenberg fetching + cache
   ntfy.py               NtfyPoller — standalone, zero internal deps
   mic.py                MicMonitor — standalone, graceful fallback
@@ -147,8 +155,15 @@ engine/
   controller.py         coordinates ntfy/mic monitoring
   emitters.py           background emitters
   types.py              type definitions
-  display.py            Display protocol (Terminal, WebSocket, Multi)
-  websocket_display.py  WebSocket server for browser clients
+  display/              Display backend system
+    __init__.py         DisplayRegistry, get_monitor
+    backends/
+      terminal.py       ANSI terminal display
+      websocket.py      WebSocket server for browser clients
+      sixel.py         Sixel graphics (pure Python)
+      null.py          headless display for testing
+      multi.py         forwards to multiple displays
+  benchmark.py          performance benchmarking tool
 ```
 
 ---
@@ -171,19 +186,25 @@ With [mise](https://mise.jdx.dev/):
 
 ```bash
 mise run test            # run test suite
-mise run test-cov        # run with coverage report
-mise run lint            # ruff check
-mise run lint-fix        # ruff check --fix
-mise run format          # ruff format
+mise run test-cov       # run with coverage report
 
-mise run run             # terminal display
-mise run run-websocket   # web display only
-mise run run-both        # terminal + web
-mise run run-client      # both + open browser
+mise run lint           # ruff check
+mise run lint-fix       # ruff check --fix
+mise run format         # ruff format
 
-mise run cmd             # C&C command interface
-mise run cmd-stats       # watch effects stats
-mise run topics-init     # initialize ntfy topics
+mise run run            # terminal display
+mise run run-websocket  # web display only
+mise run run-sixel     # sixel graphics
+mise run run-both       # terminal + web
+mise run run-client     # both + open browser
+
+mise run cmd            # C&C command interface
+mise run cmd-stats      # watch effects stats
+
+mise run benchmark      # run performance benchmarks
+mise run benchmark-json # save as JSON
+
+mise run topics-init    # initialize ntfy topics
 ```
 
 ### Testing
@@ -191,8 +212,21 @@ mise run topics-init     # initialize ntfy topics
 ```bash
 uv run pytest
 uv run pytest --cov=engine --cov-report=term-missing
+
+# Run with mise
+mise run test
+mise run test-cov
+
+# Run performance benchmarks
+mise run benchmark
+mise run benchmark-json
+
+# Run benchmark hook mode (for CI)
+uv run python -m engine.benchmark --hook
 ```
 
+Performance regression tests are in `tests/test_benchmark.py` marked with `@pytest.mark.benchmark`.
+
 ### Linting
 
 ```bash