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/display/backends/terminal.py

@@ -157,6 +157,9 @@ class TerminalDisplay:
     def cleanup(self) -> None:
         from engine.terminal import CURSOR_ON
 
+        # Disable mouse tracking if enabled
+        self.disable_mouse_tracking()
+
         # Restore normal terminal mode if raw mode was enabled
         self.set_raw_mode(False)
 
@@ -174,6 +177,24 @@ class TerminalDisplay:
         """Request quit (e.g., when Ctrl+C is pressed)."""
         self._quit_requested = True
 
+    def enable_mouse_tracking(self) -> None:
+        """Enable SGR mouse tracking mode."""
+        try:
+            # SGR mouse mode: \x1b[?1006h
+            sys.stdout.write("\x1b[?1006h")
+            sys.stdout.flush()
+        except (OSError, AttributeError):
+            pass  # Terminal might not support mouse tracking
+
+    def disable_mouse_tracking(self) -> None:
+        """Disable SGR mouse tracking mode."""
+        try:
+            # Disable SGR mouse mode: \x1b[?1006l
+            sys.stdout.write("\x1b[?1006l")
+            sys.stdout.flush()
+        except (OSError, AttributeError):
+            pass
+
     def set_raw_mode(self, enable: bool = True) -> None:
         """Enable/disable raw terminal mode for input capture.
 
@@ -192,7 +213,11 @@ class TerminalDisplay:
                 # Set raw mode
                 tty.setraw(sys.stdin.fileno())
                 self._raw_mode_enabled = True
+                # Enable mouse tracking
+                self.enable_mouse_tracking()
             elif not enable and self._raw_mode_enabled:
+                # Disable mouse tracking
+                self.disable_mouse_tracking()
                 # Restore original terminal settings
                 if self._original_termios:
                     termios.tcsetattr(
@@ -223,16 +248,35 @@ class TerminalDisplay:
                 char = sys.stdin.read(1)
 
                 if char == "\x1b":  # Escape sequence
-                    # Read next character to determine key
-                    seq = sys.stdin.read(2)
-                    if seq == "[A":
-                        keys.append("up")
-                    elif seq == "[B":
-                        keys.append("down")
-                    elif seq == "[C":
-                        keys.append("right")
-                    elif seq == "[D":
-                        keys.append("left")
+                    # Read next characters to determine key
+                    # Try to read up to 10 chars for longer sequences
+                    seq = sys.stdin.read(10)
+
+                    # PageUp: \x1b[5~
+                    if seq.startswith("[5~"):
+                        keys.append("page_up")
+                    # PageDown: \x1b[6~
+                    elif seq.startswith("[6~"):
+                        keys.append("page_down")
+                    # Arrow keys: \x1b[A, \x1b[B, etc.
+                    elif seq.startswith("["):
+                        if seq[1] == "A":
+                            keys.append("up")
+                        elif seq[1] == "B":
+                            keys.append("down")
+                        elif seq[1] == "C":
+                            keys.append("right")
+                        elif seq[1] == "D":
+                            keys.append("left")
+                        else:
+                            # Unknown escape sequence
+                            keys.append("escape")
+                    # Mouse events: \x1b[<B;X;Ym or \x1b[<B;X;YM
+                    elif seq.startswith("[<"):
+                        mouse_seq = "\x1b" + seq
+                        mouse_data = self._parse_mouse_event(mouse_seq)
+                        if mouse_data:
+                            keys.append(mouse_data)
                     else:
                         # Unknown escape sequence
                         keys.append("escape")
@@ -248,8 +292,6 @@ class TerminalDisplay:
                     keys.append("ctrl_c")
                 elif char == "\x04":  # Ctrl+D
                     keys.append("ctrl_d")
-                elif char == "\x1b":  # Escape
-                    keys.append("escape")
                 elif char.isprintable():
                     keys.append(char)
         except OSError:
@@ -257,6 +299,40 @@ class TerminalDisplay:
 
         return keys
 
+    def _parse_mouse_event(self, data: str) -> str | None:
+        """Parse SGR mouse event sequence.
+
+        Format: \x1b[<B;X;Ym (release) or \x1b[<B;X;YM (press)
+        B = button number (0=left, 1=middle, 2=right, 64=wheel up, 65=wheel down)
+        X, Y = coordinates (1-indexed)
+
+        Returns:
+            Mouse event string like "mouse:64:10:5" or None if not a mouse event
+        """
+        if not data.startswith("\x1b[<"):
+            return None
+
+        # Find the ending 'm' or 'M'
+        end_pos = data.rfind("m")
+        if end_pos == -1:
+            end_pos = data.rfind("M")
+            if end_pos == -1:
+                return None
+
+        inner = data[3:end_pos]  # Remove \x1b[< and trailing m/M
+        parts = inner.split(";")
+
+        if len(parts) >= 3:
+            try:
+                button = int(parts[0])
+                x = int(parts[1]) - 1  # Convert to 0-indexed
+                y = int(parts[2]) - 1
+                return f"mouse:{button}:{x}:{y}"
+            except ValueError:
+                pass
+
+        return None
+
     def is_raw_mode_enabled(self) -> bool:
         """Check if raw mode is currently enabled."""
         return self._raw_mode_enabled