Add Parquet export format for simulation results

[jbrodovsky]

Free Core Feature

The Free Core spec requires output in both CSV and Parquet formats to support efficient analysis in Python/MATLAB/R.

Requirements:

• Add Parquet writer using arrow/parquet crate
• Support same schema as CSV output
• Add --output-format CLI argument (csv|parquet|both)
• Preserve timestamps and all navigation solution fields
• Compress Parquet files efficiently

Acceptance Criteria:

• User can specify: strapdown-sim ... --output results.parquet --output-format parquet
• Parquet files load correctly in pandas, polars, R
• File sizes are smaller than equivalent CSV for large datasets
• Documentation updated with Parquet examples