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.Net.cs

@@ -155,6 +155,10 @@ public partial class MeshPainter
             PaintStamp3D(s.P, s.R, s.U, s.V, s.N);
             EndCpuStroke();
         }
+
+        // Accumulate this peer's stroke dirty mask so a network checkpoint marker can commit the same
+        // history entry as the owner (same stamps -> same touched maps -> COW-aligned with the owner).
+        MarkStrokePainted();
     }
 
     // --- Late-join snapshot: the whole painted state as a compressed blob ---
@@ -210,8 +214,12 @@ public partial class MeshPainter
         return true;
     }
 
-    /// <summary>Decompresses a snapshot and re-seeds the active backend with it. Caller must ensure IsPaintReady.</summary>
-    public void ApplyPaintSnapshot(byte[] compressed)
+    /// <summary>
+    /// Decompresses a snapshot, re-seeds the active backend with it, and makes it this peer's sole history
+    /// baseline under the authority's <paramref name="baselineId"/> (so subsequent checkpoint/restore
+    /// commands line up). Caller must ensure IsPaintReady.
+    /// </summary>
+    public void ApplyPaintSnapshot(byte[] compressed, int baselineId)
     {
         if (!_setupValid || compressed == null || compressed.Length == 0) return;
 
@@ -226,10 +234,8 @@ public partial class MeshPainter
 
         // Re-seed exactly like SwitchBackend, but with externally supplied maps, so the island/dilation
         // seeding path runs and the edge padding is rebuilt correctly for the loaded content.
-        if (_gpuActive) TeardownGpu();
-        else TeardownCpu();
-        SetupBackend(useGpu, canonical);
-        _hasLastScreen = false;
+        ReseedCanonical(canonical);
+        SetHistoryBaseline(canonical, baselineId);
     }
 
     private Color32[][] DecodeSnapshot(byte[] compressed)