feat(repl): Add REPL effect with HUD-style interactive interface

Implement a Read-Eval-Print Loop (REPL) effect that provides a
HUD-style overlay for interactive pipeline control.

## New Files

- engine/effects/plugins/repl.py - REPL effect plugin with command processor
- engine/display/backends/terminal.py - Added raw mode and input handling
- examples/repl_simple.py - Simple demonstration script
- tests/test_repl_effect.py - 18 comprehensive tests

## Features

### REPL Interface
- HUD-style overlay showing FPS, command history, output buffer size
- Command history navigation (Up/Down arrows)
- Command execution (Enter)
- Character input and backspace support
- Output buffer with scrolling

### Commands
- help - Show available commands
- status - Show pipeline status and metrics
- effects - List all effects in pipeline
- effect <name> <on|off> - Toggle effect
- param <effect> <param> <value> - Set parameter
- pipeline - Show current pipeline order
- clear - Clear output buffer
- quit - Show exit message

### Terminal Input Support
- Added set_raw_mode() to TerminalDisplay for capturing keystrokes
- Added get_input_keys() to read keyboard input
- Proper terminal state restoration on cleanup

## Usage

Add 'repl' to effects in your configuration:

## Testing

All 18 REPL tests pass, covering:
- Effect registration
- Command processing
- Navigation (history, editing)
- Configuration
- Rendering

## Integration

The REPL effect integrates with the existing pipeline system:
- Uses EffectPlugin interface
- Supports partial updates
- Reads metrics from EffectContext
- Can be controlled via keyboard when terminal display is in raw mode

Next steps:
- Integrate REPL input handling into pipeline_runner.py
- Add keyboard event processing loop
- Create full demo with interactive features

tests/test_repl_effect.py

@@ -0,0 +1,201 @@
+"""Tests for the REPL effect plugin."""
+
+import pytest
+from pathlib import Path
+
+from engine.effects.plugins import discover_plugins
+from engine.effects.registry import get_registry
+from engine.effects.plugins.repl import ReplEffect, REPLState
+
+
+class TestReplEffectRegistration:
+    """Tests for REPL effect registration."""
+
+    def test_repl_registered(self):
+        """REPL effect is registered in the registry."""
+        discover_plugins()
+        registry = get_registry()
+        repl = registry.get("repl")
+        assert repl is not None
+        assert repl.name == "repl"
+
+
+class TestReplEffectCreation:
+    """Tests for creating REPL effect instances."""
+
+    def test_create_repl_effect(self):
+        """Can create REPL effect instance."""
+        repl = ReplEffect()
+        assert repl.name == "repl"
+        assert repl.config.enabled is True
+        assert repl.config.intensity == 1.0
+
+    def test_repl_state(self):
+        """REPL state is initialized correctly."""
+        repl = ReplEffect()
+        assert repl.state.command_history == []
+        assert repl.state.current_command == ""
+        assert repl.state.history_index == -1
+        assert repl.state.output_buffer == []
+
+
+class TestReplEffectCommands:
+    """Tests for REPL command processing."""
+
+    @pytest.fixture(autouse=True)
+    def setup(self):
+        """Setup before each test."""
+        self.repl = ReplEffect()
+
+    def test_process_command_help(self):
+        """Help command adds help text to output."""
+        self.repl.process_command("help")
+        assert "> help" in self.repl.state.output_buffer
+        assert any(
+            "Available commands:" in line for line in self.repl.state.output_buffer
+        )
+
+    def test_process_command_status(self):
+        """Status command adds status info to output."""
+        self.repl.process_command("status")
+        assert "> status" in self.repl.state.output_buffer
+        assert any("Output lines:" in line for line in self.repl.state.output_buffer)
+
+    def test_process_command_clear(self):
+        """Clear command clears output buffer."""
+        self.repl.process_command("help")
+        initial_count = len(self.repl.state.output_buffer)
+        assert initial_count > 0
+
+        self.repl.process_command("clear")
+        assert len(self.repl.state.output_buffer) == 0
+
+    def test_process_command_unknown(self):
+        """Unknown command adds error message."""
+        self.repl.process_command("unknown_command_xyz")
+        assert "> unknown_command_xyz" in self.repl.state.output_buffer
+        assert any("Unknown command" in line for line in self.repl.state.output_buffer)
+
+    def test_command_history(self):
+        """Commands are added to history."""
+        self.repl.process_command("help")
+        self.repl.process_command("status")
+        assert len(self.repl.state.command_history) == 2
+        assert self.repl.state.command_history[0] == "help"
+        assert self.repl.state.command_history[1] == "status"
+
+    def test_current_command_cleared(self):
+        """Current command is cleared after processing."""
+        self.repl.state.current_command = "test"
+        self.repl.process_command("help")
+        assert self.repl.state.current_command == ""
+
+
+class TestReplNavigation:
+    """Tests for REPL navigation (history, editing)."""
+
+    @pytest.fixture(autouse=True)
+    def setup(self):
+        """Setup before each test."""
+        self.repl = ReplEffect()
+        self.repl.state.command_history = ["help", "status", "effects"]
+
+    def test_navigate_history_up(self):
+        """Navigate up through command history."""
+        self.repl.navigate_history(-1)  # Up
+        assert self.repl.state.history_index == 0
+        assert self.repl.state.current_command == "help"
+
+    def test_navigate_history_down(self):
+        """Navigate down through command history."""
+        self.repl.state.history_index = 0
+        self.repl.navigate_history(1)  # Down
+        assert self.repl.state.history_index == 1
+        assert self.repl.state.current_command == "status"
+
+    def test_append_to_command(self):
+        """Append character to current command."""
+        self.repl.append_to_command("h")
+        self.repl.append_to_command("e")
+        self.repl.append_to_command("l")
+        self.repl.append_to_command("p")
+        assert self.repl.state.current_command == "help"
+
+    def test_backspace(self):
+        """Remove last character from command."""
+        self.repl.state.current_command = "hel"
+        self.repl.backspace()
+        assert self.repl.state.current_command == "he"
+
+    def test_clear_command(self):
+        """Clear current command."""
+        self.repl.state.current_command = "test"
+        self.repl.clear_command()
+        assert self.repl.state.current_command == ""
+
+
+class TestReplProcess:
+    """Tests for REPL effect processing."""
+
+    @pytest.fixture(autouse=True)
+    def setup(self):
+        """Setup before each test."""
+        discover_plugins()
+        self.repl = ReplEffect()
+
+    def test_process_renders_output(self):
+        """Process renders REPL interface."""
+        buf = ["line1", "line2", "line3"]
+        from engine.effects.types import EffectContext
+
+        ctx = EffectContext(
+            terminal_width=80, terminal_height=24, scroll_cam=0, ticker_height=0
+        )
+        result = self.repl.process(buf, ctx)
+
+        assert len(result) == 24  # Should match terminal height
+        assert any("MAINLINE REPL" in line for line in result)
+        assert any("COMMANDS:" in line for line in result)
+        assert any("OUTPUT:" in line for line in result)
+
+    def test_process_with_commands(self):
+        """Process shows command output in REPL."""
+        buf = ["line1"]
+        from engine.effects.types import EffectContext
+
+        ctx = EffectContext(
+            terminal_width=80, terminal_height=24, scroll_cam=0, ticker_height=0
+        )
+        self.repl.process_command("help")
+        result = self.repl.process(buf, ctx)
+
+        # Check that command output appears in the REPL area
+        # (help output may be partially shown due to buffer size limits)
+        assert any("effects - List all effects" in line for line in result)
+
+
+class TestReplConfig:
+    """Tests for REPL configuration."""
+
+    def test_config_params(self):
+        """REPL config has expected parameters."""
+        repl = ReplEffect()
+        assert "display_height" in repl.config.params
+        assert "show_hud" in repl.config.params
+        assert repl.config.params["display_height"] == 8
+        assert repl.config.params["show_hud"] is True
+
+    def test_configure(self):
+        """Can configure REPL effect."""
+        repl = ReplEffect()
+        from engine.effects.types import EffectConfig
+
+        config = EffectConfig(
+            enabled=False,
+            intensity=0.5,
+            params={"display_height": 10, "show_hud": False},
+        )
+        repl.configure(config)
+        assert repl.config.enabled is False
+        assert repl.config.intensity == 0.5
+        assert repl.config.params["display_height"] == 10