On Sunday, residents and county officials celebrated the San Geronimo Valley Affordable Housing Association’s purchase in November of the historic Forest Knolls Trailer Court, which effectively preserved 20 units for affordable housing in perpetuity. The nonprofit bought the trailer court for $1.4 million from the county, following Marin’s purchase of it from its original owners in October 2014. Combined with the nonprofit’s purchase in 1995 of three buildings reserved as six affordable housing units for…