Derive microsimulation weights from household weights

[MaxGhenis]

Fixes #438.
Refs #384.

Summary

• Change Microsimulation.get_weights() to derive all entity weights from household_weight instead of looking up {entity}_weight variables.
• Project household weights to persons and other group entities without relying on redundant stored weight variables.
• Pass map_to through calculate_dataframe() weight lookup so mapped dataframes receive weights for the dataframe entity.

Verification

• uv run --no-sync pytest tests/core/test_microsimulation_weights.py tests/core/test_microsimulation_person_accessor.py tests/core/test_subsample_invalidates_cache.py policyengine_core/country_template/tests/test_microsimulation.py -q
• uv run --no-sync ruff format --check policyengine_core/simulations/microsimulation.py tests/core/test_microsimulation_weights.py
• uv run --no-sync ruff check policyengine_core/simulations/microsimulation.py tests/core/test_microsimulation_weights.py
• uvx --from towncrier towncrier build --draft --version 3.26.10

Documentation Review

• Durable docs beyond source docstrings are not required: docs/python_api/simulations.md uses autodoc :members:, and get_weights() now documents that weights come from household_weight.
• Impact: medium. Confidence: high.