Document expected error behavior in FRI E2F4 snapshot tests

Several FRI E2F4 test snapshots show `Error: AdapterInnerValueOverflow(...)` in their output, but the tests pass. This is confusing for anyone reviewing the test results.

**Example files:**
- `processor/src/fast/tests/snapshots/*FriE2F4*.snap`

The snapshots show errors like:
```
Error: AdapterInnerValueOverflow(...)
```

These errors are expected when `poe` equals `ZERO`. The tests verify that this error case is handled correctly.

We should add comments to the test code explaining why these errors appear in snapshots and why they're expected. The snapshots themselves are correct.

**References:**
- https://github.com/0xMiden/miden-vm/pull/2472#discussion_r2650455271
- https://github.com/0xMiden/miden-vm/pull/2472#discussion_r2650545946