Add mouse wheel and keyboard scrolling support to REPL

- Add scroll_offset to REPLState (max 50 lines)
- Modify _render_repl() to use manual scroll position
- Add scroll_output(delta) method for scroll control
- Add PageUp/PageDown keyboard support (scroll 10 lines)
- Add mouse wheel support via SGR mouse tracking
- Update HUD to show scroll percentage (like vim) and position
- Reset scroll when new output arrives
- Add tests for scroll functionality

engine/app/pipeline_runner.py

@@ -1016,8 +1016,21 @@ def run_pipeline_mode(preset_name: str = "demo", graph_config: str | None = None
                         repl_effect.navigate_history(-1)
                     elif key == "down":
                         repl_effect.navigate_history(1)
+                    elif key == "page_up":
+                        repl_effect.scroll_output(-10)  # Scroll up 10 lines
+                    elif key == "page_down":
+                        repl_effect.scroll_output(10)  # Scroll down 10 lines
                     elif key == "backspace":
                         repl_effect.backspace()
+                    elif key.startswith("mouse:"):
+                        # Mouse event format: mouse:button:x:y
+                        parts = key.split(":")
+                        if len(parts) >= 2:
+                            button = int(parts[1])
+                            if button == 64:  # Wheel up
+                                repl_effect.scroll_output(-3)  # Scroll up 3 lines
+                            elif button == 65:  # Wheel down
+                                repl_effect.scroll_output(3)  # Scroll down 3 lines
                     elif len(key) == 1:
                         repl_effect.append_to_command(key)
             # --- End REPL Input Handling ---