Fix 1 (Important): RollUp_FourLevelDeepBadQuality_ReachesRoot — proves bad quality at a
4-segment-deep leaf propagates HasBadQuality up every ancestor to the root.
Fix 2 (Important): Filter_DeepLeafMatch_RetainsAllAncestorBranches — proves filtering on
a terminal segment of a 3-level path retains all ancestor branches.
Fix 3 (Minor): BuildAttributeTree now returns roots.AsReadOnly() so the returned
IReadOnlyList<DebugTreeNode> reference is not a mutable list.
Fix 4 (Minor): Added <remarks> XML doc to BuildAttributeTree noting the caller-contract
that at most one AttributeValueChanged per AttributeName should be passed.
All 18 DebugTreeBuilder tests pass.
Joseph Doherty
69b83379d5