Dropping a PK column was already refused in both modes via the shared
do_drop_column guard; this adds end-to-end coverage on the advanced
ALTER surface (single-column + compound PK, asserting refusal for the
right reason) and documents the asymmetry that advanced-mode SQL can
create a PK-less table (SQLite's implicit rowid keys it) while simple
mode forbids it. See issue #19 comment for the full assessment.
claude@clouddev1
e44d2983ab