Update SOI long-term capital gains target to 2023

[MaxGhenis]

Summary

• Update the SOI long-term capital gains calibration parameter to IRS Publication 1304 Table 1.4A net long-term capital gain amounts for 2015, 2020, and 2023.
• Point the reference at the current IRS Publication 1304 Basic Tables Part 1 page, which lists 2023 Table 1.4A.
• Add a parameter regression test that pins the source-cell anchors and requires the 2023 anchor.

Notes

• The prior 2015 and 2020 anchors matched Schedule D taxable net gain, not the Table 1.4A net long-term capital gain column used for PUF P23250-style LTCG.
• New CBO-uprated target values are about .278T for 2024 and .686T for 2026.

Tests

• env -u UV_FROZEN uv run pytest policyengine_us/tests/test_parameter_files.py -q
• env -u UV_FROZEN uv run pytest policyengine_us/tests/test_system_import.py policyengine_us/tests/test_parameter_files.py -q
• env -u UV_FROZEN uv run ruff check policyengine_us/tests/test_parameter_files.py
• env -u UV_FROZEN uv run ruff format --check policyengine_us/tests/test_parameter_files.py
• git diff --check