Add student loan repayment calibration targets

[MaxGhenis]

Summary

• add 2025 Student Loans Company repayment amount targets by UK country and by England repayment plan
• route those targets through the current target registry and loss-matrix path using modeled student_loan_repayment
• keep the target series observational only for now, without any made-up forward projection

Target values

• England total: £5.018bn
• England Plan 1: £1.853bn
• England Plan 2: £2.778bn
• England postgraduate: £0.346bn
• England Plan 5: £0.041bn
• Scotland total: £0.203bn
• Wales total: £0.229bn
• Northern Ireland total: £0.182bn

Testing

• uvx ruff check policyengine_uk_data/targets/sources/slc_repayments.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 policyengine_uk_data/tests/test_student_loan_plan.py
• uv run --python 3.13 pytest -q policyengine_uk_data/tests/test_target_registry.py

Closes #237.