PAC_7B_cont and PAC_8B_cont missing from variables.csv

[DougManuel]

Problem

`PAC_7B_cont` and `PAC_8B_cont` have rows in `variable_details.csv` but no corresponding entry in `variables.csv`. This was introduced in PR #157 (v3-phys-activity, merged).

These are continuous derived variables (midpoint-imputed minutes) from categorical time-spent-walking/biking variables:

• `PAC_7B_cont`: Time walking to work/school (continuous), 2007–2014 PUMF
• `PAC_8B_cont`: Time biking to work/school (continuous), 2007–2014 PUMF

Fix

Add rows to `variables.csv` matching the databaseStart from variable_details:

• variableType: Continuous
• subject: Exercise
• section: Health behaviour
• units: mins
• databaseStart: cchs2007_2008_p, cchs2009_2010_p, cchs2010_p, cchs2011_2012_p, cchs2012_p, cchs2013_2014_p, cchs2014_p, cchs2009_s, cchs2010_s, cchs2012_s

Detection

Found during cchsflow-validation Check 3 (cross-file consistency) on the v3-smoking branch.