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/plugins/__init__.py

@@ -0,0 +1,26 @@
+"""
+Sideline Plugin System.
+
+This module provides the plugin framework for Sideline, allowing applications
+to extend the pipeline with custom stages, effects, and sources.
+
+Features:
+- Plugin base classes with metadata
+- Security permission system
+- Compatibility management
+- Entry point discovery
+"""
+
+from sideline.plugins.base import StagePlugin, Plugin, PluginMetadata
+from sideline.plugins.security import SecurityCapability, SecurityManager
+from sideline.plugins.compatibility import VersionConstraint, CompatibilityManager
+
+__all__ = [
+    "StagePlugin",
+    "Plugin",  # Backward compatibility alias
+    "PluginMetadata",
+    "SecurityCapability",
+    "SecurityManager",
+    "VersionConstraint",
+    "CompatibilityManager",
+]