Add in-game paint HUD bound to the mesh painter

Add a code-driven HUD that appears in paint mode and binds live to the
active MeshPainter: four channel toggles, HSV pickers for albedo and
emission, sliders for metallic/smoothness/occlusion, emission intensity
and brush size. The HUD self-assembles its own overlay canvas and event
system at runtime.

Add a ModeChanged event to GameModeManager so the HUD can follow the
active mode, and guard MeshPainter against painting through the UI.

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

Assets/Scripts/HsvColorPicker.cs

@@ -0,0 +1,124 @@
+using System;
+using UnityEngine;
+using UnityEngine.UI;
+
+/// <summary>
+/// Compact runtime HSV colour picker: a saturation/value square, a hue bar and a live
+/// swatch. Builds its own UI as children when <see cref="Build"/> is called and raises
+/// <see cref="Changed"/> on user interaction. The square is a pure-hue base with two
+/// static gradient overlays on top (white-out -> saturation, black-out -> value), so
+/// only the base tint changes per hue - no per-frame texture rebuilds. Hue is never
+/// lost while dragging the square (the square only edits S/V, the bar only edits H).
+/// </summary>
+public class HsvColorPicker : MonoBehaviour
+{
+    /// <summary>Raised with the new colour when the user moves the square or hue bar.</summary>
+    public event Action<Color> Changed;
+
+    private float _h, _s, _v, _a = 1f;
+
+    private Image _hueBase;          // pure-hue background of the SV square
+    private RectTransform _svHandle;
+    private RectTransform _hueHandle;
+    private Image _swatch;
+
+    /// <summary>Current colour (HSV -> RGB, with the preserved alpha).</summary>
+    public Color Value
+    {
+        get
+        {
+            Color c = Color.HSVToRGB(_h, _s, _v);
+            c.a = _a;
+            return c;
+        }
+    }
+
+    /// <summary>Shows <paramref name="c"/> without raising <see cref="Changed"/> (external set).</summary>
+    public void SetColor(Color c)
+    {
+        Color.RGBToHSV(c, out _h, out _s, out _v);
+        _a = c.a;
+        Refresh();
+    }
+
+    /// <summary>Builds the picker (square + hue bar + swatch) under this object, top-left aligned.</summary>
+    public void Build(float squareSize, float hueHeight)
+    {
+        // Saturation / value square: base hue + two gradient overlays, top-left of this rect.
+        var sv = PaintHud.NewRect("SV", transform);
+        TopLeft(sv, new Vector2(0f, 0f), new Vector2(squareSize, squareSize));
+        _hueBase = PaintHud.AddImage(sv.gameObject, Color.red); // raycast target for the drag relay
+
+        PaintHud.AddOverlay(sv, PaintHud.SaturationSprite());
+        PaintHud.AddOverlay(sv, PaintHud.ValueSprite());
+
+        sv.gameObject.AddComponent<UiPointerSurface>().Picked += OnSvPicked;
+
+        _svHandle = PaintHud.NewRect("Handle", sv);
+        _svHandle.sizeDelta = new Vector2(12f, 12f);
+        var svHandleImg = PaintHud.AddImage(_svHandle.gameObject, Color.white);
+        svHandleImg.raycastTarget = false;
+
+        // Hue bar below the square.
+        var hue = PaintHud.NewRect("Hue", transform);
+        TopLeft(hue, new Vector2(0f, -(squareSize + 6f)), new Vector2(squareSize, hueHeight));
+        var hueImg = hue.gameObject.AddComponent<Image>();
+        hueImg.sprite = PaintHud.HueSprite();
+        hueImg.type = Image.Type.Simple;
+
+        hue.gameObject.AddComponent<UiPointerSurface>().Picked += OnHuePicked;
+
+        _hueHandle = PaintHud.NewRect("Handle", hue);
+        _hueHandle.sizeDelta = new Vector2(4f, hueHeight + 4f);
+        var hueHandleImg = PaintHud.AddImage(_hueHandle.gameObject, Color.white);
+        hueHandleImg.raycastTarget = false;
+
+        // Swatch, to the right of the square.
+        var sw = PaintHud.NewRect("Swatch", transform);
+        TopLeft(sw, new Vector2(squareSize + 10f, 0f), new Vector2(34f, 34f));
+        _swatch = PaintHud.AddImage(sw.gameObject, Color.white);
+        _swatch.raycastTarget = false;
+
+        Refresh();
+    }
+
+    private void OnSvPicked(Vector2 p)
+    {
+        _s = p.x;
+        _v = p.y;
+        Refresh();
+        Changed?.Invoke(Value);
+    }
+
+    private void OnHuePicked(Vector2 p)
+    {
+        _h = p.x;
+        Refresh();
+        Changed?.Invoke(Value);
+    }
+
+    private void Refresh()
+    {
+        if (_hueBase) _hueBase.color = Color.HSVToRGB(_h, 1f, 1f);
+        if (_svHandle)
+        {
+            _svHandle.anchorMin = _svHandle.anchorMax = new Vector2(_s, _v);
+            _svHandle.anchoredPosition = Vector2.zero;
+        }
+        if (_hueHandle)
+        {
+            _hueHandle.anchorMin = _hueHandle.anchorMax = new Vector2(_h, 0.5f);
+            _hueHandle.anchoredPosition = Vector2.zero;
+        }
+        if (_swatch) _swatch.color = Color.HSVToRGB(_h, _s, _v);
+    }
+
+    /// <summary>Anchors a child to this rect's top-left corner with the given offset and size.</summary>
+    private static void TopLeft(RectTransform rt, Vector2 offset, Vector2 size)
+    {
+        rt.anchorMin = rt.anchorMax = new Vector2(0f, 1f);
+        rt.pivot = new Vector2(0f, 1f);
+        rt.anchoredPosition = offset;
+        rt.sizeDelta = size;
+    }
+}