verify.sh: count the background jobs instead of hardcoding the tally

Adding the social-meta gate made eight jobs where the loop said seven: it exited a job early, the EXIT trap wiped $TMP under the still-running matproof, and its half-written .res read as a spurious coordscover FAIL.

▶ Play it: Stationary Weather — a long-form genre — put the phone in your pocket and it keeps playing https://stellate.app/?genre=ambient&seed=7934&path=2103.13280,2122.13313,2084.13313&xdur=13.122

05f2f2b https://github.com/aboard-io/stellate/commit/05f2f2be40075846d3c4d7c84d4dce04ca36e997