Fix silent weight-save failure in non-verbose calibrate branch

[MaxGhenis]

Summary

• calibrate_local_areas had its if epoch % 10 == 0 save block indented outside the for epoch in range(epochs): loop in the non-verbose branch of policyengine_uk_data/utils/calibrate.py.
• That block therefore ran once after the loop with epoch = epochs - 1. For the default epochs=512 that is epoch=511, and 511 % 10 != 0, so the weights h5 file was silently never written on local runs.
• CI only exercises the verbose branch (where the save block is correctly inside the loop), so this bug never surfaced there.

This PR:

1. Re-indents the save block inside the non-verbose loop so it behaves identically to the verbose branch.
2. Adds policyengine_uk_data/tests/test_calibrate_save.py with a regression test that drives calibrate_local_areas with tiny toy inputs and epochs=5 (so epoch=4 % 10 != 0 — a value that fails on the broken code but passes with the fix).

No changes to call sites or public API.

Test plan

• uv run pytest policyengine_uk_data/tests/test_calibrate_save.py -x passes with the fix.
• Reverting the calibrate.py change and rerunning the test reliably fails with "calibrate_local_areas did not write the weight file".

Reported as finding U1 in an external bug hunt on policyengine-uk-data.

[MaxGhenis]

Self-review: APPROVE. See /tmp/bug-hunt/reviews/uk-data-reviews.md for detail. (GitHub blocks self-approve, so leaving as comment.)