Define circuit group subtrees

docs/spec/09-distribution-board-components-and-protection-groups.md

@@ -663,8 +663,8 @@ Acceptance:
 ### D. Group Numbering and Circuit Moves
 
 Status: In progress. Group renumbering D1 and deterministic circuit-move
-planning and persistent moves D2 are complete; populated deletion D3 remains
-pending.
+planning and persistent moves D2 plus the deletion snapshot D3a are complete;
+persistent populated deletion D3b remains pending.
 
 - implement nested identifier generation
 - support same-category cross-group circuit moves
@@ -711,6 +711,16 @@ Implemented in D2b:
 - Undo/Redo uses the stored inverse move and late failures restore source
   group, BMK and position
 
+Implemented in D3a:
+
+- one validated subtree snapshot combines the group, optional group components
+  and their protection devices, complete circuits and their protection devices,
+  and all circuit device rows
+- linked-project-device and override metadata remain part of every row snapshot
+- ownership and BMK uniqueness are checked across the complete subtree
+- the snapshot derives the confirmation summary for upstream protection, RCD,
+  circuit count and device-row count
+
 Acceptance:
 
 - target identifiers use highest suffix plus one