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

examples/repl_simple.py

@@ -0,0 +1,78 @@
+#!/usr/bin/env python3
+"""
+Simple REPL Demo - Just shows the REPL effect rendering
+
+This is a simpler version that doesn't require raw terminal mode,
+just demonstrates the REPL effect rendering.
+"""
+
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from engine.effects.plugins import discover_plugins
+from engine.effects.registry import get_registry
+from engine.effects.types import EffectContext
+from engine.pipeline.hybrid_config import PipelineConfig
+
+
+def main():
+    """Run simple REPL demo."""
+    print("Simple REPL Demo")
+    print("=" * 50)
+
+    # Discover plugins
+    discover_plugins()
+
+    # Create a simple pipeline with REPL
+    config = PipelineConfig(
+        source="headlines",
+        effects=[{"name": "repl", "intensity": 1.0}],
+        display={"backend": "null"},
+    )
+
+    pipeline = config.to_pipeline(viewport_width=80, viewport_height=24)
+
+    # Initialize pipeline
+    if not pipeline.initialize():
+        print("Failed to initialize pipeline")
+        return
+
+    # Get the REPL effect
+    repl_effect = None
+    for stage in pipeline._stages.values():
+        if hasattr(stage, "_effect") and stage._effect.name == "repl":
+            repl_effect = stage._effect
+            break
+
+    if not repl_effect:
+        print("REPL effect not found")
+        return
+
+    # Get the EffectContext for REPL
+    # Note: In a real pipeline, the EffectContext is created per-stage
+    # For this demo, we'll simulate by adding commands
+
+    # Add some commands to the output
+    repl_effect.process_command("help")
+    repl_effect.process_command("status")
+    repl_effect.process_command("effects")
+    repl_effect.process_command("pipeline")
+
+    # Execute pipeline to see REPL output
+    result = pipeline.execute([])
+
+    if result.success:
+        print("\nPipeline Output:")
+        print("-" * 50)
+        for line in result.data:
+            print(line)
+        print("-" * 50)
+        print(f"\n✓ Successfully rendered {len(result.data)} lines")
+    else:
+        print(f"✗ Pipeline error: {result.error}")
+
+
+if __name__ == "__main__":
+    main()