docs(readme): catalogue scenario 11 and close its scratch self-check

The suite-coverage table and the concurrency-scenario prose enumerate
every scenario by number and by the property it pins, so adding one
without listing it leaves the catalogue claiming complete coverage it
no longer has. PR #9 updated the same table for the same reason when it
added 8b/8c/8d/10; this follows it.

Also adds the closing no-scratch-left-behind check that 8a and ALIAS
end with. Scenario 11 is last in the file, so nothing downstream would
have caught a leaked staging path or reader marker.

scripts/seed-target-dir-selftest.sh

@@ -720,6 +720,9 @@ ok "both sides of the race completed"
 grep -q "readers: waiting for 1 in-flight clone(s) of snapshot-${INTLK_BASE}" "$scratch/logPubIntlk" \
   || { tail -40 "$scratch/logPubIntlk"; fail "the publisher never reported waiting on the consumer's reader marker — the interlock did not observably engage"; }
 ok "the publisher observably waited on the consumer's own reader marker before reclaiming the rotated snapshot generation"
+leftovers=$(find "$root" -maxdepth 1 \( -name '.stage-*' -o -name '.reading-*' -o -name '.publish-*' \) -print)
+[ -z "$leftovers" ] || fail "scratch left behind: ${leftovers}"
+ok "no staging, reader-marker or deferred-generation scratch left behind"
 rm -f "$scratch/bin/cp"
 
 echo