bva.py in this folder is a tested budget-vs-actual script: from one P&L CSV it reports a month and the year to date. Derive from it a trailing-twelve-month variance script, ttm.py, in this folder. The window is the twelve months ending at --period (for --period 2025-06 that is 2024-07 to 2025-06), so the script reads both years' files, data/pnl_fy2024.csv and data/pnl_fy2025.csv. Keep bva.py's JSON shape, its display formats, its exit codes and its identity check; change only what the new window needs. Leave bva.py as it is. When ttm.py works, run it with --period 2025-06 and write its output to output/ttm.json.
