Ability to ship/source from multiple sources in 1 shipment

### Description (*)
We have 3 sources in 1 stock. All 3 sources are local and I can walk there ... and pick the items. Source 1 is in our store wh, source 2 is in the large warehouse and source 3 is used for incoming items.

If our stock looks like this
source 1: 1
source 2: 1
source 3: 1
then salable qty = 3

An order comes in from 1 customer ordering the 3 last items. Now we cannot make 1 shipment sourcing from source 1, 2 and 3 ...

### Expected behavior (*)
- Use source selection algoritm and apply it across sources respecting the priority and salable qty.
- The user can create 1 shipment utilizing all 3 sources

In short: one should be able to ship by sourcing from several different sources in 1 shipment. In real life sources may not be geographically located for away, but they might all be close to each other. You then want Magento to sum all sources to ship it in 1 go. If you need 10 items, find them amongst the sources based on some prio, source them and ship them in 1 go.


### Benefits
1 shipment instead of 3. It is almost ridiculous to create 3 shipments and un-doable.

### Additional information



No files, tests, or entry points are named in the issue. Start by locating the source-selection algorithm and shipment creation flow, then determine how priority and salable quantity are represented across sources. Done means one shipment can fulfill an order by combining inventory from multiple sources while respecting source priority.

Written by the indexing model from the issue text.