feat: Implement Sideline plugin system with consistent terminology

This commit implements the Sideline/Mainline split with a clean plugin architecture:

## Core Changes

### Sideline Framework (New Directory)
- Created  directory containing the reusable pipeline framework
- Moved pipeline core, controllers, adapters, and registry to
- Moved display system to
- Moved effects system to
- Created plugin system with security and compatibility management in
- Created preset pack system with ASCII art encoding in
- Added default font (Corptic) to
- Added terminal ANSI constants to

### Mainline Application (Updated)
- Created  for Mainline stage component registration
- Updated  to register Mainline stages at startup
- Updated  as a compatibility shim re-exporting from sideline

### Terminology Consistency
- : Base class for all pipeline components (sources, effects, displays, cameras)
- : Base class for distributable plugin packages (was )
- : Base class for visual effects (was )
- Backward compatibility aliases maintained for existing code

## Key Features
- Plugin discovery via entry points and explicit registration
- Security permissions system for plugins
- Compatibility management with semantic version constraints
- Preset pack system for distributable configurations
- Default font bundled with Sideline (Corptic.otf)

## Testing
- Updated tests to register Mainline stages before discovery
- All StageRegistry tests passing

Note: This is a major refactoring that separates the framework (Sideline) from the application (Mainline), enabling Sideline to be used by other applications.

sideline/__init__.py

@@ -0,0 +1,85 @@
+"""
+Sideline - A modular pipeline framework for real-time terminal visualization.
+
+Sideline provides a Stage-based pipeline architecture with capability-based
+dependency resolution for building real-time visualization applications.
+
+Features:
+- Stage-based pipeline execution with DAG dependency resolution
+- Capability-based dependency injection
+- Display backends (Terminal, WebSocket, Null, etc.)
+- Effect plugin system with param bindings
+- Sensor framework for real-time input
+- Canvas and Camera for 2D rendering
+
+Example:
+    from sideline.pipeline import Pipeline, PipelineConfig, StageRegistry
+
+    pipeline = Pipeline(PipelineConfig(source="custom", display="terminal"))
+    pipeline.add_stage("source", MyDataSourceStage())
+    pipeline.add_stage("display", StageRegistry.create("display", "terminal"))
+    pipeline.build().initialize()
+
+    result = pipeline.execute(initial_data)
+"""
+
+__version__ = "0.1.0"
+
+# Re-export core components for convenience
+from sideline.pipeline import (
+    Pipeline,
+    PipelineConfig,
+    PipelineContext,
+    Stage,
+    StageRegistry,
+)
+
+from sideline.display import Display, DisplayRegistry
+
+from sideline.effects import Effect, EffectPlugin, EffectRegistry
+
+from sideline.plugins import (
+    StagePlugin,
+    Plugin,  # Backward compatibility
+    PluginMetadata,
+    SecurityCapability,
+    SecurityManager,
+    VersionConstraint,
+    CompatibilityManager,
+)
+
+from sideline.preset_packs import (
+    PresetPack,
+    PresetPackMetadata,
+    PresetPackManager,
+    PresetPackEncoder,
+)
+
+__all__ = [
+    # Pipeline
+    "Pipeline",
+    "PipelineConfig",
+    "PipelineContext",
+    "Stage",
+    "StageRegistry",
+    # Display
+    "Display",
+    "DisplayRegistry",
+    # Effects
+    "Effect",  # Primary class name
+    "EffectPlugin",  # Backward compatibility alias
+    "EffectRegistry",
+    # Plugins
+    "StagePlugin",
+    "Plugin",  # Backward compatibility alias
+    "PluginMetadata",
+    "SecurityCapability",
+    "SecurityManager",
+    "VersionConstraint",
+    "CompatibilityManager",
+    # Preset Packs
+    "PresetPack",
+    "PresetPackMetadata",
+    "PresetPackManager",
+    "PresetPackEncoder",
+]