Problem: tbl_deep_extend merges user platforms on top of defaults, so
all four default platforms survive even when the user only configures a
subset. The picker then shows platforms the user never intended to use.
Solution: before the deep merge, prune any default platform not present
in the user's platforms table. This preserves per-platform default
filling (the user doesn't have to re-specify every field) while ensuring
only explicitly configured platforms appear.
Barrett Ruth
06f72bbe2b