fix: Implement ViewportFilterStage to prevent FontStage performance regression with large datasets

## Summary

Fixed critical performance issue where demo/poetry presets would hang for 10+ seconds due to FontStage rendering all 1438+ headline items instead of just the visible ~5 items.

## Changes

### Core Fix: ViewportFilterStage
- New pipeline stage that filters items to only those fitting in the viewport
- Reduces 1438 items → ~5 items (288x reduction) before FontStage
- Prevents expensive PIL font rendering operations on items that won't be displayed
- Located: engine/pipeline/adapters.py:348-403

### Pipeline Integration
- Updated app.py to add ViewportFilterStage before FontStage for headlines/poetry sources
- Ensures correct data flow: source → viewport_filter → font → camera → effects → display
- ViewportFilterStage depends on 'source' capability, providing pass-through filtering

### Display Protocol Enhancement
- Added is_quit_requested() and clear_quit_request() method signatures to Display protocol
- Documented as optional methods for backends supporting keyboard input
- Already implemented by pygame backend, now formally part of protocol

### Debug Infrastructure
- Added MAINLINE_DEBUG_DATAFLOW environment variable logging throughout pipeline
- Logs stage input/output types and data sizes to stderr (when flag enabled)
- Verified working: 1438 → 5 item reduction shown in debug output

### Performance Testing
- Added pytest-benchmark (v5.2.3) as dev dependency for statistical benchmarking
- Created comprehensive performance regression tests (tests/test_performance_regression.py)
- Tests verify:
  - ViewportFilterStage filters 2000 items efficiently (<1ms)
  - FontStage processes filtered items quickly (<50ms)
  - 288x performance improvement ratio maintained
  - Pipeline doesn't hang with large datasets
- All 523 tests passing, including 7 new performance tests

## Performance Impact

**Before:** FontStage renders all 1438 items per frame → 10+ second hang
**After:** FontStage renders ~5 items per frame → sub-second execution

Real-world impact: Demo preset now responsive and usable with news sources.

## Testing

- Unit tests: 523 passed, 16 skipped
- Regression tests: Catch performance degradation with large datasets
- E2E verification: Debug logging confirms correct pipeline flow
- Benchmark suite: Statistical performance tracking enabled

engine/pipeline/controller.py

@@ -255,6 +255,18 @@ class Pipeline:
         1. Execute all non-overlay stages in dependency order
         2. Apply overlay stages on top (sorted by render_order)
         """
+        import os
+        import sys
+
+        debug = os.environ.get("MAINLINE_DEBUG_DATAFLOW") == "1"
+
+        if debug:
+            print(
+                f"[PIPELINE.execute] Starting with data type: {type(data).__name__ if data else 'None'}",
+                file=sys.stderr,
+                flush=True,
+            )
+
         if not self._initialized:
             self.build()
 
@@ -303,8 +315,30 @@ class Pipeline:
             stage_start = time.perf_counter() if self._metrics_enabled else 0
 
             try:
+                if debug:
+                    data_info = type(current_data).__name__
+                    if isinstance(current_data, list):
+                        data_info += f"[{len(current_data)}]"
+                    print(
+                        f"[STAGE.{name}] Starting with: {data_info}",
+                        file=sys.stderr,
+                        flush=True,
+                    )
+
                 current_data = stage.process(current_data, self.context)
+
+                if debug:
+                    data_info = type(current_data).__name__
+                    if isinstance(current_data, list):
+                        data_info += f"[{len(current_data)}]"
+                    print(
+                        f"[STAGE.{name}] Completed, output: {data_info}",
+                        file=sys.stderr,
+                        flush=True,
+                    )
             except Exception as e:
+                if debug:
+                    print(f"[STAGE.{name}] ERROR: {e}", file=sys.stderr, flush=True)
                 if not stage.optional:
                     return StageResult(
                         success=False,