refactor(D.2b): ElementReader review fixes — defensive Children copy + sentinel doc

- Merge: defensive copy `new List<ElementInfo>(derived.Children)` so a
  later mutation of the merged result or the input can't corrupt the other
- Merge: add comment on Width/Height 0-sentinel (Plan-1 safe; Plan-2
  limitation and float?-upgrade path documented inline)
- Test: replace mid-sentence "Wait —" authoring trace in
  EdgeFlagsToAnchors_ValueThree_FallsBackToTopLeft with a clean
  conclusion-first summary of the value-3 mapping rule

9/9 ElementReaderTests pass; 0 build errors.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/AcDream.App/UI/Layout/ElementReader.cs

@@ -145,6 +145,11 @@ public static class ElementReader
             Type      = derived.Type      != 0 ? derived.Type      : base_.Type,
             X         = derived.X,
             Y         = derived.Y,
+            // NOTE: 0 is the "not set, inherit from base" sentinel for Width/Height. This
+            // diverges from the format doc §12 rule 2 ("derived W/H win even if zero") but is
+            // indistinguishable for Plan 1 (all base elements are zero-size Type-12 prototypes).
+            // If a real zero-size derived element ever needs to override a non-zero base in
+            // Plan 2, switch Width/Height to float? + null-coalescing (and update Tasks 3-5).
             Width     = derived.Width     != 0 ? derived.Width     : base_.Width,
             Height    = derived.Height    != 0 ? derived.Height    : base_.Height,
             Left      = derived.Left,
@@ -154,7 +159,10 @@ public static class ElementReader
             ReadOrder = derived.ReadOrder,
             FontDid   = derived.FontDid   != 0 ? derived.FontDid   : base_.FontDid,
             // Children come from the derived element's own tree, not the base prototype's.
-            Children  = derived.Children,
+            // Defensive copy: prevent a later mutation of either the merged result or the input
+            // from corrupting the other. Safe for the Task-5 flow (derived.Children is fully
+            // populated by the recursive importer BEFORE Merge is called and never mutated after).
+            Children  = new List<ElementInfo>(derived.Children),
         };
         // Start with base StateMedia as defaults, then let derived entries override.
         m.StateMedia = new Dictionary<string, (uint, int)>(base_.StateMedia);