Persist painted maps and HUD layout across sessions

Add MeshPainter.Persistence.cs: auto-save the three canonical paint maps
(albedo, MSO, emission) as PNGs under persistentDataPath on quit/play-stop
and auto-load them as the backend seed on start, so painting survives
sessions. Reuses the existing ReadCanonical/SetupBackend seed round-trip;
no new color-space code.

Add PaintHud.Persistence.cs: auto-save draggable panel positions and the
widget-bound painter values to a small JSON layout file, restored on start
via the painter -> SyncFromPainter -> widgets path. Complete SyncFromPainter
to also cover the spray sliders.

Assets/Scripts/MeshPainter.Persistence.cs

@@ -0,0 +1,164 @@
+using System.IO;
+using UnityEngine;
+using UnityEngine.SceneManagement;
+
+public partial class MeshPainter
+{
+    // --- Persistence: save the painted maps to disk and reload them next session ---
+
+    [Header("Persistence")]
+    [Tooltip("Save the painted maps to disk on quit / play-stop and reload them on the next " +
+             "start, so the painting persists across sessions. Off = paint is volatile (as before).")]
+    public bool persistPaint = true;
+
+    [Tooltip("Save file identifier. Empty = derived from the active scene + object name. Set " +
+             "explicitly to keep a save stable across object renames or to share one save.")]
+    public string saveId = "";
+
+    // Raw, bit-exact map dump: 'CHMP' magic + version + width + height + map count, then each
+    // map's canonical Color32 bytes. Raw (not PNG) avoids any sRGB/alpha-premultiply ambiguity
+    // and round-trips the exact canonical bytes ReadCanonical produces.
+    private const uint PaintFileMagic = 0x504D4843; // 'CHMP' little-endian
+    private const int PaintFileVersion = 1;
+
+    private bool _savedOnTeardown; // guards against a double save across OnApplicationQuit + OnDestroy
+
+    private static string PaintSaveDir => Path.Combine(Application.persistentDataPath, "MeshPaint");
+
+    private string PaintSavePath()
+    {
+        string key = string.IsNullOrEmpty(saveId)
+            ? SanitizeKey(SceneManager.GetActiveScene().name + "_" + gameObject.name)
+            : SanitizeKey(saveId);
+        return Path.Combine(PaintSaveDir, key + ".paint");
+    }
+
+    private static string SanitizeKey(string s)
+    {
+        foreach (char c in Path.GetInvalidFileNameChars()) s = s.Replace(c, '_');
+        return s.Replace(' ', '_');
+    }
+
+    /// <summary>Reads all maps from the active backend (canonical layout) and writes them to the save file.</summary>
+    public void SavePaint()
+    {
+        if (!persistPaint || !_setupValid) return;
+
+        try
+        {
+            Directory.CreateDirectory(PaintSaveDir);
+            int n = textureSize * textureSize;
+
+            using (var fs = new FileStream(PaintSavePath(), FileMode.Create, FileAccess.Write))
+            using (var w = new BinaryWriter(fs))
+            {
+                w.Write(PaintFileMagic);
+                w.Write(PaintFileVersion);
+                w.Write(textureSize);
+                w.Write(textureSize);
+                w.Write(MAP_COUNT);
+
+                var bytes = new byte[n * 4];
+                for (int mi = 0; mi < MAP_COUNT; mi++)
+                {
+                    Color32[] px = ReadCanonical(mi);
+                    int count = Mathf.Min(n, px.Length);
+                    for (int i = 0; i < count; i++)
+                    {
+                        int o = i * 4;
+                        bytes[o] = px[i].r;
+                        bytes[o + 1] = px[i].g;
+                        bytes[o + 2] = px[i].b;
+                        bytes[o + 3] = px[i].a;
+                    }
+                    w.Write(bytes, 0, count * 4);
+                }
+            }
+        }
+        catch (System.Exception e)
+        {
+            Debug.LogWarning($"[MeshPainter] Saving paint failed: {e.Message}", this);
+        }
+    }
+
+    /// <summary>
+    /// Loads the saved maps in canonical layout for seeding, or null if none / incompatible.
+    /// Per-map null entries are tolerated by both backends (they fall back to the material seed).
+    /// </summary>
+    private Color32[][] LoadPaintMaps()
+    {
+        if (!persistPaint) return null;
+
+        string path = PaintSavePath();
+        if (!File.Exists(path)) return null;
+
+        try
+        {
+            using (var fs = new FileStream(path, FileMode.Open, FileAccess.Read))
+            using (var r = new BinaryReader(fs))
+            {
+                if (r.ReadUInt32() != PaintFileMagic) return null;
+                int ver = r.ReadInt32();
+                int w = r.ReadInt32();
+                int h = r.ReadInt32();
+                int maps = r.ReadInt32();
+
+                // Resolution / version change -> ignore the save rather than smear mismatched
+                // data; the brush re-seeds from the material instead.
+                if (ver != PaintFileVersion || w != textureSize || h != textureSize)
+                {
+                    Debug.LogWarning($"[MeshPainter] Saved paint ignored (got {w}x{h} v{ver}, expected " +
+                                     $"{textureSize}x{textureSize} v{PaintFileVersion}). Re-seeding from material.", this);
+                    return null;
+                }
+
+                int n = textureSize * textureSize;
+                var result = new Color32[MAP_COUNT][];
+                int load = Mathf.Min(maps, MAP_COUNT);
+                for (int mi = 0; mi < load; mi++)
+                {
+                    byte[] raw = r.ReadBytes(n * 4);
+                    if (raw.Length != n * 4) break; // truncated file -> remaining maps stay seeded
+
+                    var px = new Color32[n];
+                    for (int i = 0; i < n; i++)
+                    {
+                        int o = i * 4;
+                        px[i] = new Color32(raw[o], raw[o + 1], raw[o + 2], raw[o + 3]);
+                    }
+                    result[mi] = px;
+                }
+                return result;
+            }
+        }
+        catch (System.Exception e)
+        {
+            Debug.LogWarning($"[MeshPainter] Loading paint failed: {e.Message}", this);
+            return null;
+        }
+    }
+
+    /// <summary>Saves once during teardown (OnApplicationQuit and OnDestroy both call this; only the first writes).</summary>
+    private void SavePaintOnTeardown()
+    {
+        if (_savedOnTeardown) return;
+        _savedOnTeardown = true;
+        SavePaint();
+    }
+
+    /// <summary>Deletes this painter's save file (next start re-seeds from the material). Editor convenience.</summary>
+    [ContextMenu("Clear Saved Paint")]
+    public void ClearSavedPaint()
+    {
+        try
+        {
+            string path = PaintSavePath();
+            if (File.Exists(path)) File.Delete(path);
+            Debug.Log($"[MeshPainter] Cleared saved paint: {path}", this);
+        }
+        catch (System.Exception e)
+        {
+            Debug.LogWarning($"[MeshPainter] Clearing saved paint failed: {e.Message}", this);
+        }
+    }
+}