Possible to preserve `//include/@merge` for output SDFormat files?

## Desired behavior
Per #749 and VC discussion, `//include/@merge=true` will not be preserved round-trip.
Would be nice to have it.

## Alternatives considered
N/A

## Implementation suggestion
Annotate included elements w/ stack of indicators about which `//include/@merge` element produced it.
Upon emitting the elements, remove them, and ensure original `//include` element is preserved.

## Additional context

\cc @azeey @jennuine

Start by reviewing issue #749 and the referenced VC discussion, then trace how included elements are expanded and how output SDFormat files are emitted. Done means `//include/@merge=true` survives a round trip while the resulting output remains valid SDFormat.

Written by the indexing model from the issue text.