Add game mode manager with player-control and paint modes

Introduce an extensible game mode system toggled with Tab:
- Player Control: WASD movement with always-on mouse camera orbit
- Paint: player frozen, left-click paints, right-click holds to orbit

Add GameMode base class, GameModeManager, and the two concrete modes.
Refactor controlled scripts to expose clean state flags instead of
handling input themselves: ThirdPersonOrbitInput.LookMode,
PlayerCharacterController.ControlEnabled, MeshPainter.PaintingEnabled.

Also fix camera zoom: remove the obsolete /120 wheel normalization so
one scroll notch maps to one zoomSpeed step under the new Input System.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/GameModeManager.cs

@@ -0,0 +1,117 @@
+using System.Collections.Generic;
+using UnityEngine;
+#if ENABLE_INPUT_SYSTEM
+using UnityEngine.InputSystem;
+#endif
+
+/// <summary>
+/// Central state manager that switches the game between high-level modes.
+/// Currently two modes exist:
+///   0. Player Control - WASD movement, camera orbits freely with the mouse.
+///   1. Paint          - player stands still, left mouse paints the mesh, the
+///      camera orbits only while the right mouse button is held.
+///
+/// Pressing the switch key (Tab) cycles to the next registered mode, so adding
+/// more modes later automatically extends the cycle.
+///
+/// The manager owns references to the systems a mode may toggle (player
+/// controller, orbit camera input, mesh painter) and exposes them to the modes.
+/// Modes are plain C# classes derived from <see cref="GameMode"/>; register
+/// additional ones in <see cref="BuildModes"/>.
+///
+/// Attach this to a single, persistent GameObject in the scene.
+/// </summary>
+[DisallowMultipleComponent]
+public class GameModeManager : MonoBehaviour
+{
+    [Header("Controlled Systems")]
+    [Tooltip("Player movement controller. Empty = found automatically in the scene.")]
+    public PlayerCharacterController player;
+
+    [Tooltip("Third-person orbit camera input. Empty = found automatically in the scene.")]
+    public ThirdPersonOrbitInput cameraInput;
+
+    [Tooltip("Mesh painter. Empty = found automatically in the scene.")]
+    public MeshPainter painter;
+
+    [Header("Switching")]
+    [Tooltip("Key that cycles to the next mode.")]
+#if ENABLE_INPUT_SYSTEM
+    public Key switchKey = Key.Tab;
+#else
+    public KeyCode switchKey = KeyCode.Tab;
+#endif
+
+    [Tooltip("Index of the mode that is active at startup (0 = Player Control).")]
+    public int startModeIndex = 0;
+
+    // --- accessors used by the modes ---
+    public PlayerCharacterController Player => player;
+    public ThirdPersonOrbitInput CameraInput => cameraInput;
+    public MeshPainter Painter => painter;
+
+    private readonly List<GameMode> _modes = new List<GameMode>();
+    private int _activeIndex = -1;
+
+    /// <summary>The currently active mode (null before startup).</summary>
+    public GameMode ActiveMode =>
+        (_activeIndex >= 0 && _activeIndex < _modes.Count) ? _modes[_activeIndex] : null;
+
+    private void Awake()
+    {
+        // Auto-resolve references so the manager works with minimal wiring.
+        if (player == null) player = FindFirstObjectByType<PlayerCharacterController>();
+        if (cameraInput == null) cameraInput = FindFirstObjectByType<ThirdPersonOrbitInput>();
+        if (painter == null) painter = FindFirstObjectByType<MeshPainter>();
+
+        BuildModes();
+    }
+
+    private void Start()
+    {
+        if (_modes.Count > 0)
+            SwitchTo(Mathf.Clamp(startModeIndex, 0, _modes.Count - 1));
+    }
+
+    /// <summary>Registers all available modes. Add new modes here.</summary>
+    private void BuildModes()
+    {
+        _modes.Clear();
+        _modes.Add(new PlayerControlMode());
+        _modes.Add(new PaintMode());
+
+        foreach (var m in _modes)
+            m.Bind(this);
+    }
+
+    private void Update()
+    {
+        if (_modes.Count == 0) return;
+
+        if (WasSwitchPressed())
+            SwitchTo((_activeIndex + 1) % _modes.Count);
+
+        ActiveMode?.Tick();
+    }
+
+    /// <summary>Activates the mode at <paramref name="index"/> (Exit old, Enter new).</summary>
+    public void SwitchTo(int index)
+    {
+        if (index < 0 || index >= _modes.Count || index == _activeIndex) return;
+
+        ActiveMode?.Exit();
+        _activeIndex = index;
+        ActiveMode.Enter();
+
+        Debug.Log($"[GameModeManager] Mode -> {ActiveMode.DisplayName}", this);
+    }
+
+    private bool WasSwitchPressed()
+    {
+#if ENABLE_INPUT_SYSTEM
+        return Keyboard.current != null && Keyboard.current[switchKey].wasPressedThisFrame;
+#else
+        return Input.GetKeyDown(switchKey);
+#endif
+    }
+}