Individual ambiguity for number of island >=4

**Describe the bug**
For four islands {A, B, C, D} an individual that migrates from A -> B -> C, looks identical to one A -> D -> C if migration history is only tracked by number of hops.
This might cause issues with population bookkeeping.

Start by tracing how migration history is represented when an individual moves across islands, focusing on the population bookkeeping described in the issue. Reproduce the A→B→C and A→D→C cases, then define and test behavior that keeps those histories distinguishable.

Written by the indexing model from the issue text.