Make simulation input exports safe by default

[MaxGhenis]

Fixes #417.
Fixes #418.

Summary

• Default Simulation.to_input_dataframe() and Simulation.to_input_dict() to export only structurally input variables populated through set_input on the current branch.
• Preserve the historical all-known-values export path with include_computed_variables=True.
• Add Simulation.true_input_variables / get_input_variables(include_computed_variables=False) for callers that need the safe source-input set.

Verification

• uv run --no-sync pytest tests/core/test_simulations.py tests/core/test_subsample_invalidates_cache.py -q
• uv run --no-sync ruff format --check policyengine_core/simulations/simulation.py tests/core/test_simulations.py
• uv run --no-sync ruff check policyengine_core/simulations/simulation.py tests/core/test_simulations.py
• uvx --from towncrier towncrier build --draft --version 3.26.9

Documentation Review

• Not required beyond docstrings: docs/python_api/simulations.md uses autodoc :members:, so the changed public methods and defaults are surfaced from the source docstrings.
• Impact: medium. Confidence: high.