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.Backend.cs

@@ -35,6 +35,19 @@ public partial class MeshPainter
         Debug.Log($"[MeshPainter] Backend switched -> {(toGpu ? "GPU" : "CPU")} painting.", this);
     }
 
+    /// <summary>
+    /// Tears down the active backend and rebuilds it from externally supplied canonical maps (load /
+    /// late-join snapshot / undo). Runs the island/dilation seeding path so edge padding is rebuilt for
+    /// the new content. The input arrays are never mutated (SetupBackend clones its seed).
+    /// </summary>
+    private void ReseedCanonical(Color32[][] canonical)
+    {
+        if (_gpuActive) TeardownGpu();
+        else TeardownCpu();
+        SetupBackend(useGpu, canonical);
+        _hasLastScreen = false;
+    }
+
     /// <summary>Reads a map from the active backend into canonical (material) layout.</summary>
     private Color32[] ReadCanonical(int mi)
     {