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>

tests/AcDream.App.Tests/UI/Layout/ElementReaderTests.cs

@@ -66,13 +66,11 @@ public class ElementReaderTests
     }
 
     /// <summary>
-    /// Value 3 = floating/centered between both far edges on that axis.
-    /// Both LeftEdge=3 and RightEdge=3 → neither Left nor Right are set by the
-    /// near/stretch rules. The result is only Right+Bottom (the "far" semantics).
-    /// Specifically: left=3 → not Left (3 is not 1 or 4); right=3 → Right (3 is not 2 or 4, skip).
-    /// Wait — value 3 means "pinned to BOTH far edges" per format doc §4. Re-check the
-    /// mapping rule: Right anchor fires on right==2 || right==4, NOT on right==3.
-    /// So value 3 on LeftEdge, TopEdge, RightEdge, BottomEdge → no flags set → default Left|Top.
+    /// Value 3 = floating/centered between both far edges on that axis (format doc §4).
+    /// The anchor mapping fires on near-pin (1) and stretch (4) for Left/Top, and on
+    /// far-pin (2) and stretch (4) for Right/Bottom — value 3 matches none of these rules.
+    /// Therefore all-3 edge flags contribute no anchor bits and fall through to the
+    /// Left|Top default (pin top-left, fixed size).
     /// This test covers that corner case (element 0x100004A9 — expand-detail overlay).
     /// </summary>
     [Fact]