parseBoolFalseDefault returned false only for explicit falsy literals
(including the empty string) and true otherwise, but the ComposeEnabled
call site negated it. The double error cancelled for an unset variable
(empty -> false -> !false -> true) but inverted every explicit value:
WORKER_COMPOSE_ENABLED=true disabled the subsystem while =false enabled
it. Rename the helper to parseBoolTrueDefault, drop the empty string
from the falsy set so unset stays on, and drop the negation. Add a
table-driven regression test pinning unset/true/1/yes -> on and
false/0/no/off (any case, trimmed) -> off.
dde720eb44