Shell script drives the Dockerfile builder stage across PG versions,
capturing pass/fail + timing per version. Makefile targets: test-matrix,
test-pg%, test-matrix-clean. Also runs standalone DE reader test in the
builder stage to catch compiler-version regressions.
Fix pork chop grid test: add ORDER BY to CROSS JOIN (optimizer chooses
different join nesting across PG versions, reordering rows).
