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.

engine/pipeline/__init__.py

@@ -1,50 +1,32 @@
 """
-Unified Pipeline Architecture.
+Unified Pipeline Architecture (Compatibility Shim).
 
-This module provides a clean, dependency-managed pipeline system:
-- Stage: Base class for all pipeline components
-- Pipeline: DAG-based execution orchestrator
-- PipelineParams: Runtime configuration for animation
-- PipelinePreset: Pre-configured pipeline configurations
-- StageRegistry: Unified registration for all stage types
+This module re-exports the pipeline architecture from Sideline for backward
+compatibility with existing Mainline code. New code should import directly
+from sideline.pipeline.
 
-The pipeline architecture supports:
-- Sources: Data providers (headlines, poetry, pipeline viz)
-- Effects: Post-processors (noise, fade, glitch, hud)
-- Displays: Output backends (terminal, pygame, websocket)
-- Cameras: Viewport controllers (vertical, horizontal, omni)
-
-Example:
-    from engine.pipeline import Pipeline, PipelineConfig, StageRegistry
-
-    pipeline = Pipeline(PipelineConfig(source="headlines", display="terminal"))
-    pipeline.add_stage("source", StageRegistry.create("source", "headlines"))
-    pipeline.add_stage("display", StageRegistry.create("display", "terminal"))
-    pipeline.build().initialize()
-
-    result = pipeline.execute(initial_data)
+Note: This module is deprecated and will be removed in future versions.
 """
 
-from engine.pipeline.controller import (
+# Re-export from sideline for backward compatibility
+from sideline.pipeline import (
     Pipeline,
     PipelineConfig,
-    PipelineRunner,
-    create_default_pipeline,
-    create_pipeline_from_params,
-)
-from engine.pipeline.core import (
     PipelineContext,
     Stage,
     StageConfig,
     StageError,
     StageResult,
-)
-from engine.pipeline.params import (
-    DEFAULT_HEADLINE_PARAMS,
-    DEFAULT_PIPELINE_PARAMS,
-    DEFAULT_PYGAME_PARAMS,
     PipelineParams,
+    StageRegistry,
+    discover_stages,
+    register_camera,
+    register_display,
+    register_effect,
+    register_source,
 )
+
+# Re-export from engine.pipeline.presets (Mainline-specific)
 from engine.pipeline.presets import (
     DEMO_PRESET,
     FIREHOSE_PRESET,
@@ -57,34 +39,40 @@ from engine.pipeline.presets import (
     get_preset,
     list_presets,
 )
-from engine.pipeline.registry import (
-    StageRegistry,
-    discover_stages,
-    register_camera,
-    register_display,
-    register_effect,
-    register_source,
+
+# Re-export from sideline.pipeline.params
+from sideline.pipeline.params import (
+    DEFAULT_HEADLINE_PARAMS,
+    DEFAULT_PIPELINE_PARAMS,
+    DEFAULT_PYGAME_PARAMS,
+)
+
+# Re-export additional functions from sideline.pipeline
+from sideline.pipeline import (
+    create_default_pipeline,
+    create_pipeline_from_params,
+    PipelineRunner,
 )
 
 __all__ = [
-    # Core
+    # Core (from sideline)
     "Stage",
     "StageConfig",
     "StageError",
     "StageResult",
     "PipelineContext",
-    # Controller
+    # Controller (from sideline)
     "Pipeline",
     "PipelineConfig",
     "PipelineRunner",
     "create_default_pipeline",
     "create_pipeline_from_params",
-    # Params
+    # Params (from sideline)
     "PipelineParams",
     "DEFAULT_HEADLINE_PARAMS",
     "DEFAULT_PIPELINE_PARAMS",
     "DEFAULT_PYGAME_PARAMS",
-    # Presets
+    # Presets (from engine)
     "PipelinePreset",
     "PRESETS",
     "DEMO_PRESET",
@@ -96,7 +84,7 @@ __all__ = [
     "get_preset",
     "list_presets",
     "create_preset_from_params",
-    # Registry
+    # Registry (from sideline)
     "StageRegistry",
     "discover_stages",
     "register_source",