A composite [pk] entry inside an Indexes block (e.g. (a, b) [pk]) was
silently dropped: models.Index has no way to represent a primary key,
so the attribute was parsed and ignored, producing neither a PK nor a
meaningful index. It's now converted into a PrimaryKeyConstraint.
Also, Column.Sequence was never set by the DBML reader, so composite
PKs assembled from column-level [pk] attributes fell back to
alphabetical Name sorting instead of declaration order. Columns now
get a per-table sequence counter reflecting the order they were
declared.