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/Prefabs/PaintHudPanel.prefab

@@ -3527,7 +3527,7 @@ RectTransform:
   m_AnchorMin: {x: 0, y: 1}
   m_AnchorMax: {x: 1, y: 1}
   m_AnchoredPosition: {x: 0, y: 0}
-  m_SizeDelta: {x: 0, y: 0}
+  m_SizeDelta: {x: 100, y: 0}
   m_Pivot: {x: 0.5, y: 1}
 --- !u!114 &6115439559885354559
 MonoBehaviour:
@@ -3542,10 +3542,10 @@ MonoBehaviour:
   m_Name: 
   m_EditorClassIdentifier: UnityEngine.UI::UnityEngine.UI.VerticalLayoutGroup
   m_Padding:
-    m_Left: 6
-    m_Right: 6
-    m_Top: 6
-    m_Bottom: 6
+    m_Left: 4
+    m_Right: 4
+    m_Top: 4
+    m_Bottom: 4
   m_ChildAlignment: 0
   m_Spacing: 3
   m_ChildForceExpandWidth: 1