EP-DESIGNER: editable designer end-to-end (columns/types/keys/indexes -> .sql)

[ilijevicdenis]

Part of #112 (DESIGNER).

M5 shipped read-only round-trip; make the designer editable end-to-end (columns, types, keys, indexes) writing back through the existing SqlEmitter round-trip.

Source: docs/SSDT_PARITY_BACKLOG.md §EP-DESIGNER.