fix(postprocess): suppress nested moves under demoted ancestor moves

[HarshK97]

Problem

1. When an ancestor container move gets demoted to a subtree Delete + Insert (like a distant cross-scope move), Move actions on its descendant nodes were still surviving in Pass 2, causing conflicting move highlights inside an already-deleted subtree.
2. In Pass 2, normalizeMovesByStructure was re-running shouldDemoteMove while demoteMoveToDelIns was actively mutating the node mappings, making demotion decisions dependent on map mutation order.

What Changed

• Recorded demoted moves in a toDemote map and their descendants in a demotedDescendants set during Pass 1, so Pass 2 never re-evaluates demotion checks against a mutating mapping.
• Skipped nested Move actions in Pass 2 if the node is in demotedDescendants, since the ancestor is already replaced as a full subtree Delete + Insert.
• Flattened the action loop in Pass 2 into an early nil guard and a clean switch a.Type.
• Added a unit test in internal/postprocess/normalize_test.go verifying that nested moves under a demoted ancestor move are suppressed and only the ancestor's Delete + Insert survive.