Peeking a queue changes its next message

### Feature Summary

`PriorityGroup.peek()` advances the shared round-robin cursor while skipping empty subqueues. A read-only peek can therefore change which message a later `get()` returns.

Before: peek skips an empty queue -> shared cursor advances -> later get returns a different queue

After: peek scans with a local cursor -> shared cursor stays unchanged -> later get preserves round-robin order

Expected behavior:

Peeking should report the next available message without changing queue selection state or the order of a later get.

Reproduction evidence:

Create two same-priority subqueues, enqueue only in the second, and call `peek()`. Then enqueue in the first and call `get()`.

Observed on `70c21145887920528d7d5540e3fb790b43e8b759`:

```text
cursor_before_peek=0
peek=second
cursor_after_peek=1
next_after_peek=second
next_without_peek=first
```

Version and commit evidence:

1.3.0-incubating-SNAPSHOT (main)

**Commit Hash (Optional)**

`70c21145887920528d7d5540e3fb790b43e8b759`

### Proposed Solution or Design

After: peek scans with a local cursor -> shared cursor stays unchanged -> later get preserves round-robin order

Expected behavior:

### Affected Area

Workflow Engine (Amber)