`IndependentConfig` synthesis raises "Cliques must be unique."

## Summary

Calling `dpsynth.generate(...)` with `discrete_config=IndependentConfig()` fails
with a `ValueError` from `mbi.CliqueVector.expand`. MST and AIM work on the same
input; only INDEPENDENT is affected.

## Environment

- Linux, Python 3.12, current `mbi` (`git+https://github.com/ryan112358/mbi.git`)

## Reproduction

```python
import pandas as pd
import dpsynth
from dpsynth import domain, discrete_mechanisms as dm

df = pd.DataFrame({
"a": ["x", "y", "x", "y"] * 50,
"b": [0, 1, 1, 0] * 50,
})
domains = {
"a": domain.CategoricalAttribute(possible_values=["x", "y"]),
"b": domain.CategoricalAttribute(possible_values=[0, 1]),
}
dpsynth.generate(df, domains, epsilon=1.0, delta=1e-5,
discrete_config=dm.IndependentConfig())
```

```text
ValueError: Cliques must be unique.
File ".../dpsynth/discrete_mechanisms/independent.py", line 73, in run_mechanism
potentials = potentials.expand([m.clique for m in measurements])
File ".../mbi/clique_vector.py", line 55, in __attrs_post_init__
raise ValueError("Cliques must be unique.")
```

## Cause

In `independent.run_mechanism`, `measurements` starts from
`initial_measurements` — the one-way marginals that
`data_generation_v2.generate` always passes in — and then the loop appends a
freshly measured one-way marginal for **every** attribute. So `measurements`
contains each one-way clique twice.

When `initial_potentials is not None` (e.g. the empty `CliqueVector` returned by
`constraints.get_initial_parameters` even when there are no cross-attribute
constraints), line 73 calls
`potentials.expand([m.clique for m in measurements])` with that duplicated
clique list. Current `mbi` requires unique cliques in `expand`, so it raises.

## Suggested fix

De-duplicate the clique list (order-preserving) before calling `expand`. The
measurements passed to `mirror_descent` are left unchanged, so estimation and
privacy accounting are unaffected. PR attached.