The SQL parser (`extract_sql`) previously only extracted foreign key
relationships defined inline within CREATE TABLE column definitions.
FK constraints added via ALTER TABLE ... ADD CONSTRAINT ... FOREIGN KEY
... REFERENCES were silently ignored.
Additionally, `_obj_name()` only read the first identifier child of
object_reference nodes, so schema-qualified names like `Sales.Customer`
were truncated to just `Sales`.
Changes:
- Add `alter_table` handler to `walk()` that extracts FK edges from
ALTER TABLE ... ADD CONSTRAINT ... FOREIGN KEY ... REFERENCES
- Fix `_obj_name()` to read the full object_reference text, preserving
schema-qualified names (e.g. `Sales.Customer`)
- Fix inline FK resolution in create_table and _walk_from_refs to use
full object_reference text instead of first identifier only
zuyeyang