Preflight checkpoint recalibration output root

[anth-volk]

Summary

Fixes #183.

• Preflight pe_us_recalibrate_from_checkpoint --output-root before starting checkpoint recalibration.
• Require --output-root to already exist, be a directory, and pass best-effort writable/searchable permission checks.
• Avoid writing a sentinel/probe file or creating the output directory during preflight.
• Keep later artifact writes using the preflighted output directory.

Tests

• ruff check src/microplex_us/pipelines/pe_us_recalibrate_from_checkpoint.py tests/pipelines/test_recalibrate_from_checkpoint.py
• git diff --check
• PYTHONPATH=/Users/administrator/Documents/PolicyEngine/microplex-us/src PYTHONDONTWRITEBYTECODE=1 /Users/administrator/Documents/PolicyEngine/worktrees/microplex-us/fix-pe-rebuild-smoke-issues/.venv/bin/python -m pytest -q -p no:cacheprovider tests/pipelines/test_recalibrate_from_checkpoint.py
• PYTHONPATH=/Users/administrator/Documents/PolicyEngine/microplex-us/src PYTHONDONTWRITEBYTECODE=1 /Users/administrator/Documents/PolicyEngine/worktrees/microplex-us/fix-pe-rebuild-smoke-issues/.venv/bin/python -m microplex_us.pipelines.pe_us_recalibrate_from_checkpoint --help

Note: --help exited 0, with sandbox Arrow CPU-info warnings.