Pin Python version in R wrapper setup

[PavelMakarchuk]

Summary

• setup_policyengine() now constrains the virtualenv to Python >=3.10,<3.14, matching the policyengine-us PyPI requirement
• On machines with only Python 3.14 (current stable), setup previously failed silently at the pip install step
• Adds clear error messages directing users to install Python 3.12 if no compatible version is found

Context

The Living Wage Calculator team bypassed setup_policyengine() entirely and wrote an 8-step manual installation guide — largely because setup_policyengine() doesn't handle the Python version constraint. On a fresh machine with Python 3.14, reticulate::virtualenv_create() picks up 3.14, then pip install policyengine-us fails because policyengine-us requires <3.14.

Changes

In r-package/policyenginetaxsim/R/setup.R:

• Added .PE_PYTHON_VERSION constant (">=3.10,<3.14") matching the policyengine-us constraint
• virtualenv_create() now passes version = .PE_PYTHON_VERSION to select a compatible Python
• Pre-checks for compatible Python using virtualenv_starter() before attempting venv creation
• Error messages now explain the version requirement and suggest brew install python@3.12

Closes #707

Test plan

• Verify setup_policyengine(force = TRUE) works on machine with Python 3.12/3.13
• Verify error message is clear on machine with only Python 3.14
• Verify policyengine_calculate_taxes() works end-to-end after setup

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