Flatten infrastructure_snapshot_cid in flatten_bom()

[JEJodesty]

docs/BOM.md documents this gap directly: flatten_bom() (cats/network/__init__.py) currently only
fetches bom.plant_snapshot_cid back out into flat_bom['plant'] for human-readable inspection;
bom.infrastructure_snapshot_cid isn't flattened the same way, so today it's only reachable by
ipfs cat-ing it directly out of the raw bom dict.

See docs/TODO.md's "infrastructure_snapshot_cid isn't flattened yet" ToDo entry.

Dependencies:

• None — bom.infrastructure_snapshot_cid is already produced by Service.execute()
  (cats/service/__init__.py); this is purely about surfacing it in flatten_bom()'s output shape.

AC:

• flatten_bom() fetches bom.infrastructure_snapshot_cid and surfaces it under
  flat_bom['infrastructure'], mirroring how flat_bom['plant'] is populated today
• docs/BOM.md's note about this gap is updated/removed once fixed