Raise test coverage from 73% to 84%

[SoundGoof]

Summary

• Raise overall test coverage from 73% to 84% (verified via make coverage in a Python 2.7 container, since this repo doesn't run under Python 3)
• Add tests/TestStatistics.py and tests/TestDiff.py, covering lib/statistics.py and lib/diff.py (previously untested, 0%)
• Extend tests/TestFirewall.py with explicit-option (s=/c=/w=/l=/p=), version-suffix, flow-prefix, and error-path tests, taking lib/firewall.py to 100%
• Add a hall-position test to tests/TestSeatmap.py, taking lib/location.py to 99%
• Fix a real, pre-existing bug: 9ca1d4d ("remove padding on vertical rows") also silently dropped the even() rounding call in location.switch_locations for both branches, not just the padding it described — multi-table (scaled) layouts were producing fractional pixel coordinates for switches instead of rounded-even integers. Restored the rounding and updated TestSeatmap's expected values to match, which also fixes TestSeatmap.testSwitchLocationWithMixedLayout, which was failing on master.

Test plan

• make coverage passes clean (exit 0) in a python:2.7 container, 73% → 84% total
• lib/diff.py, lib/statistics.py, lib/firewall.py at 100%; lib/location.py at 99% (remaining line is a trailing continue that CPython/coverage.py can't trace as a separate line, not an actual gap)
• All existing tests still pass, including the previously-broken TestSeatmap.testSwitchLocationWithMixedLayout