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/fonts/__init__.py

@@ -0,0 +1,38 @@
+"""
+Sideline font configuration.
+
+Provides default fonts for block letter rendering.
+"""
+
+import os
+from pathlib import Path
+
+# Directory containing Sideline fonts
+FONTS_DIR = Path(__file__).parent
+
+# Default font for block letter rendering
+DEFAULT_FONT = FONTS_DIR / "Corptic.otf"
+
+# Font size for default rendering
+DEFAULT_FONT_SIZE = 32
+
+
+def get_default_font_path() -> str:
+    """Get path to default font file."""
+    if DEFAULT_FONT.exists():
+        return str(DEFAULT_FONT)
+    raise FileNotFoundError(f"Default font not found: {DEFAULT_FONT}")
+
+
+def get_default_font_size() -> int:
+    """Get default font size."""
+    return DEFAULT_FONT_SIZE
+
+
+__all__ = [
+    "get_default_font_path",
+    "get_default_font_size",
+    "DEFAULT_FONT",
+    "DEFAULT_FONT_SIZE",
+    "FONTS_DIR",
+]