[FEATURE] `merge_grids`: add `"merge_ids"` mode that only adds new assets

[jaapschoutenalliander]

Describe the feature request

Extend Grid.merge() / merge_grids() to support a merge mode where overlapping assets (by id) are silently skipped, and only assets from the other grid that are absent in the current grid are added.

Current behavior

mode="keep_ids" appends all arrays from the other grid and then calls check_ids(). If any id collision is found, it raises:

ValueError: Asset ids are not unique after merging! Use mode='recalculate_ids' to avoid this.

There is no way to merge two datasets that share a common subset of assets without recalculating IDs.

Requested behavior

Add a new mode, e.g. mode="merge_ids", that for each array in other_grid, only appends rows whose id is not already present in the corresponding array of the current grid.

Motivation / use case

When two grids are built independently but represent overlapping parts of the same network (e.g. a connecting topology shared between scenarios), merging with "recalculate_ids" breaks cross-references, while "keep_ids" fails. The desired semantics is "add what's new, ignore what's already there".

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