Duplicate channels can crash range shuffle routing

### Feature Summary

Range shuffle deduplicates downstream receivers but divides the key range by the original channel count. Repeated topology channels can therefore produce a receiver index beyond the deduplicated receiver list and crash on a valid key.

Before: four channels to two unique receivers compute index 3 and raise IndexError

After: the range uses two unique receivers and routes the maximum key to receiver B

Range calculation should use the same unique receiver list used for routing, so every valid key maps to an existing receiver.

Reproduction evidence:

Configure range 0 through 9 with channels to A, A, A, and B, then route key 9.

Observed result:

```text
channels=4 receivers=2 keys_per_receiver=3 index_for_9=3
IndexError: list index out of range
```

Version and commit evidence:

1.3.0-incubating-SNAPSHOT (main)

**Commit Hash (Optional)**

`50321e403c82df299a13deb50a7f9849dd93bdba`

### Proposed Solution or Design

After: the range uses two unique receivers and routes the maximum key to receiver B

### Affected Area

Workflow Engine (Amber)