Split repos/compare.py below the 800-line growth cap

dokimi/repos/compare.py is 807 lines, over the 800-line growth cap that tests/test_growth_ratchet.py enforces.

#336 (rule ledger) adds 7 lines to it, so that PR carries a scripts/growth_baseline.json entry for compare.py with this issue as the reason. The entry is a ratchet exception, not a fix.

Follow-up: split compare.py into modules under 800 lines in its own PR, then remove the baseline entry.