Merger

Combines two independent sets of edits (left and right, both relative to the same base) into one merge result, per MERGE.md's design direction. Built directly on EditDetector: nodes edited by only one side are trivially safe to apply; nodes edited by both sides are reconciled attribute-by-attribute, so (per the running HelloWorld2A/HelloWorld2B example) one side moving an object and the other restyling it combine cleanly, while two sides changing the same attribute differently is reported as a Conflict, not silently resolved either way. A node deleted by one side and updated by the other is also a Conflict (per Kaining's own catalogue of expected outcomes, 06-delete-vs-modify in MERGE.md - this deliberately disagrees with Lindholm's stated default of letting deletion win silently).

Deliberately narrow, matching EditDetector's own scope: only attribute-level content is reconciled, not child order/position - PcsMerger handles that; ThreeWayMerger combines the two.