Add Ctrl+Z/Ctrl+Y undo/redo with command-based network sync

Pointer-based per-painter history of the canonical maps with copy-on-write
sharing (only maps touched by a stroke are cloned). A stroke (press..release)
is one step; a Load resets the history to the loaded maps as the sole baseline.

Network sync replicates undo/redo as tiny checkpoint/restore commands instead
of broadcasting full map snapshots: every peer runs the same COW history keyed
by owner-assigned checkpoint ids and reproduces restores locally. A peer that
cannot reproduce a target (undo across its join boundary) requests a targeted
snapshot from the owner as fallback.

Hardening: ordered send FIFO keeps stamps + checkpoints + restores in
production order (a marker never overtakes the stamps it finalizes); stamp
overflow carries over to the next frame instead of being decimated, so remote
content matches the owner; remote always inserts the checkpoint id to stay
1:1 aligned. Owner-authoritative snapshot fallback replaces the silent
host no-op. Adds a logSync toggle (default on) tracing when each
checkpoint/restore/snapshot is sent, applied, or falls back.

Also includes paintings-panel layout tuning in PaintHudPanel.prefab
(inspector edits).

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

Assets/Scripts/MeshPainter.cs

@@ -303,6 +303,7 @@ public partial class MeshPainter : MonoBehaviour
         // HUD library (MeshPainter.Persistence), never auto-loaded on start.
         SetupBackend(useGpu, _seedCanonical);
         ConfigureMaterialPbr();
+        InitUndoHistory(); // baseline = material seed, captured before any stroke
 
         EnsureProceduralMask();
         ResolveActiveMask();
@@ -592,10 +593,22 @@ public partial class MeshPainter : MonoBehaviour
         // Painting disabled by the game mode (e.g. player-control mode): do nothing.
         if (!PaintingEnabled)
         {
+            EndStrokeIfActive(); // finalize an in-progress stroke if the mode was left mid-paint
             _hasLastScreen = false;
             return;
         }
 
+        // Undo / redo (Ctrl+Z / Ctrl+Y): only here, since PaintingEnabled is true only on the local
+        // owner in paint mode. Commit any in-progress stroke first so it becomes its own history step.
+        GetUndoRedoInput(out bool wantUndo, out bool wantRedo);
+        if (wantUndo || wantRedo)
+        {
+            EndStrokeIfActive();
+            if (wantUndo) Undo();
+            else Redo();
+            return;
+        }
+
         if (updateColliderEveryFrame && _skinned)
         {
             BakeColliderMesh();
@@ -617,6 +630,7 @@ public partial class MeshPainter : MonoBehaviour
         // entirely (no painting, no brush preview ring while active).
         if (pipette)
         {
+            EndStrokeIfActive(); // switching to the eyedropper ends any active stroke
             if (pressed)
             {
                 if (!_pickHeld) PickAtScreen(screenPos);
@@ -635,6 +649,7 @@ public partial class MeshPainter : MonoBehaviour
 
         if (!pressed)
         {
+            EndStrokeIfActive(); // mouse released -> commit the stroke as one undo step
             _hasLastScreen = false;
             return;
         }
@@ -717,6 +732,7 @@ public partial class MeshPainter : MonoBehaviour
         else PaintStamp3D(pLocal, radiusLocal, u, v, nLocal);
 
         EmitStamp(pLocal, radiusLocal, u, v, nLocal);
+        MarkStrokePainted(); // a stamp landed -> this stroke is undoable
     }
 
     /// <summary>