Add PLA and ADG SLC targets

[MaxGhenis]

Summary

• add official SLC targets for Parents' Learning Allowance and Adult Dependants' Grant from Table 4C(i)
• route those new count and spend targets through the calibration loss matrix
• bump policyengine-uk to >=2.85.0 so the new variables are available in uk-data CI

Target values

From Student support for higher education in England 2025, Table 4C (i) for academic year 2024/25 -> 2025:

• slc/parents_learning_allowance_recipients: 99,645
• slc/parents_learning_allowance_spend: £181,421,659
• slc/adult_dependants_grant_recipients: 18,611
• slc/adult_dependants_grant_spend: £55,364,917

Current-weight baseline against the new targets

Using enhanced_frs_2023_24.h5 at 2025 with the released policyengine-uk 2.85.0:

• PLA recipients: 40.1k vs 99.6k
• PLA spend: £81.1m vs £181.4m
• ADG recipients: 5.2k vs 18.6k
• ADG spend: £18.4m vs £55.4m

So current weights are materially low on both products, which is exactly why these targets belong in the objective.

Validation

• uvx ruff check policyengine_uk_data/targets/sources/slc.py policyengine_uk_data/targets/compute/other.py policyengine_uk_data/targets/compute/__init__.py policyengine_uk_data/targets/build_loss_matrix.py policyengine_uk_data/tests/test_student_loan_targets.py
• uv run --python 3.13 pytest -q policyengine_uk_data/tests/test_student_loan_targets.py