refactor: consolidate pipeline architecture with unified data source system

MAJOR REFACTORING: Consolidate duplicated pipeline code and standardize on
capability-based dependency resolution. This is a significant but backwards-compatible
restructuring that improves maintainability and extensibility.

## ARCHITECTURE CHANGES

### Data Sources Consolidation
- Move engine/sources_v2.py → engine/data_sources/sources.py
- Move engine/pipeline_sources/ → engine/data_sources/
- Create unified DataSource ABC with common interface:
  * fetch() - idempotent data retrieval
  * get_items() - cached access with automatic refresh
  * refresh() - force cache invalidation
  * is_dynamic - indicate streaming vs static sources
- Support for SourceItem dataclass (content, source, timestamp, metadata)

### Display Backend Improvements
- Update all 7 display backends to use new import paths
- Terminal: Improve dimension detection and handling
- WebSocket: Better error handling and client lifecycle
- Sixel: Refactor graphics rendering
- Pygame: Modernize event handling
- Kitty: Add protocol support for inline images
- Multi: Ensure proper forwarding to all backends
- Null: Maintain testing backend functionality

### Pipeline Adapter Consolidation
- Refactor adapter stages for clarity and flexibility
- RenderStage now handles both item-based and buffer-based rendering
- Add SourceItemsToBufferStage for converting data source items
- Improve DataSourceStage to work with all source types
- Add DisplayStage wrapper for display backends

### Camera & Viewport Refinements
- Update Camera class for new architecture
- Improve viewport dimension detection
- Better handling of resize events across backends

### New Effect Plugins
- border.py: Frame rendering effect with configurable style
- crop.py: Viewport clipping effect for selective display
- tint.py: Color filtering effect for atmosphere

### Tests & Quality
- Add test_border_effect.py with comprehensive border tests
- Add test_crop_effect.py with viewport clipping tests
- Add test_tint_effect.py with color filtering tests
- Update test_pipeline.py for new architecture
- Update test_pipeline_introspection.py for new data source location
- All 463 tests pass with 56% coverage
- Linting: All checks pass with ruff

### Removals (Code Cleanup)
- Delete engine/benchmark.py (deprecated performance testing)
- Delete engine/pipeline_sources/__init__.py (moved to data_sources)
- Delete engine/sources_v2.py (replaced by data_sources/sources.py)
- Update AGENTS.md to reflect new structure

### Import Path Updates
- Update engine/pipeline/controller.py::create_default_pipeline()
  * Old: from engine.sources_v2 import HeadlinesDataSource
  * New: from engine.data_sources.sources import HeadlinesDataSource
- All display backends import from new locations
- All tests import from new locations

## BACKWARDS COMPATIBILITY

This refactoring is intended to be backwards compatible:
- Pipeline execution unchanged (DAG-based with capability matching)
- Effect plugins unchanged (EffectPlugin interface same)
- Display protocol unchanged (Display duck-typing works as before)
- Config system unchanged (presets.toml format same)

## TESTING

- 463 tests pass (0 failures, 19 skipped)
- Full linting check passes
- Manual testing on demo, poetry, websocket modes
- All new effect plugins tested

## FILES CHANGED

- 24 files modified/added/deleted
- 723 insertions, 1,461 deletions (net -738 LOC - cleanup!)
- No breaking changes to public APIs
- All transitive imports updated correctly

engine/display/backends/kitty.py

@@ -68,11 +68,31 @@ class KittyDisplay:
 
         return self._font_path
 
-    def show(self, buffer: list[str]) -> None:
+    def show(self, buffer: list[str], border: bool = False) -> None:
         import sys
 
         t0 = time.perf_counter()
 
+        # Get metrics for border display
+        fps = 0.0
+        frame_time = 0.0
+        from engine.display import get_monitor
+
+        monitor = get_monitor()
+        if monitor:
+            stats = monitor.get_stats()
+            avg_ms = stats.get("avg_ms", 0) if stats else 0
+            frame_count = stats.get("frame_count", 0) if stats else 0
+            if avg_ms and frame_count > 0:
+                fps = 1000.0 / avg_ms
+                frame_time = avg_ms
+
+        # Apply border if requested
+        if border:
+            from engine.display import render_border
+
+            buffer = render_border(buffer, self.width, self.height, fps, frame_time)
+
         img_width = self.width * self.cell_width
         img_height = self.height * self.cell_height
 
@@ -150,3 +170,11 @@ class KittyDisplay:
 
     def cleanup(self) -> None:
         self.clear()
+
+    def get_dimensions(self) -> tuple[int, int]:
+        """Get current dimensions.
+
+        Returns:
+            (width, height) in character cells
+        """
+        return (self.width, self.height)