Add deterministic map conflict detection to Y.Map writes

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What this task asks for

Add strict, deterministic conflict detection for Y.Map key writes with collect and error policies. Add strict, deterministic conflict detection for Y.Map-style key writes so ambiguous or overlapping operations are detected early, reported clearly, and optionally block updates before they partially apply. Conflicts must be detected for set-set and delete-set on the same key within the same transaction or merged update when mapConflictPolicy is collect or error. Conflicts involving Yjs types or subdocs must be marked as ambiguous, either by setting conflict.type to ambiguous or by exposing an ambiguous boolean flag. The policy is configured via the Y.Doc constructor options as new Y.Doc({ mapConflictPolicy: 'allow'|'collect'|'error' }). The thrown error must expose an err.conflicts array. The summary must also include an overall count as count or total.