dasymetric validate_disaggregation materializes full arrays without memory guard

[brendancol]

Describe the bug

validate_disaggregation() at lines 689-690 calls _to_numpy() on both the result and zones arrays, which .compute()s dask arrays. No memory guard.

This is a validation function (not a core computation), so materialization is expected, but it should fail fast on oversized inputs rather than letting the OOM killer handle it.

Expected behavior

Add a memory guard before the .compute() calls.