Fix reversed parameter value history intervals

[anth-volk]

Fixes #499

Summary

• return Parameter.parameter_values from oldest to newest
• derive inclusive, non-overlapping end dates from the following effective start
• retain Core's first effective value when duplicate starts are present
• document the public history contract and add US, UK, consecutive-day, and reform-conversion regressions

Validation

• uv run --frozen pytest tests/test_parameter_values.py tests/test_parametric_reforms.py tests/test_models.py tests/test_dict_reforms_on_simulation.py -q — 67 passed
• uv run --frozen make format — passed
• uv run --frozen make lint — passed
• uv run --frozen mypy src/policyengine/core/parameter.py — passed
• full installed US and UK catalog audit — zero interval violations

Full integration and dataset-backed tests were not run.