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

REPL_USAGE.md

@@ -108,6 +108,22 @@ Pipeline effects:
 Effect 'repl' set to off
 ```
 
+## Scrolling Support
+
+The REPL output buffer supports scrolling through command history:
+
+**Keyboard Controls:**
+- **PageUp** - Scroll up 10 lines
+- **PageDown** - Scroll down 10 lines
+- **Mouse wheel up** - Scroll up 3 lines
+- **Mouse wheel down** - Scroll down 3 lines
+
+**Scroll Features:**
+- **Scroll percentage** shown in HUD (like vim, e.g., "50%")
+- **Scroll position** shown in output line (e.g., "(5/20)")
+- **Auto-reset** - Scroll resets to bottom when new output arrives
+- **Max buffer** - 50 lines (excluding empty lines)
+
 ## Notes
 
 - The REPL effect needs a content source to overlay on (e.g., headlines, poetry, empty)