The database layer now honours the ColumnSpec constraint
fields end to end, ahead of the grammar that lets users type
them.
- `do_create_table` emits ` NOT NULL` / ` UNIQUE` / ` DEFAULT
<literal>` per column via the new `column_constraints_sql`
helper (the default literal bound against the column's type).
- `ReadColumn` gains `default_sql`, read from
`pragma_table_info.dflt_value`; `schema_to_ddl` emits it, so
the rebuild-table primitive preserves DEFAULT — it already
preserved NOT NULL / UNIQUE.
- `ColumnDescription` gains `unique` / `default`;
`do_describe_table` now sources columns from `read_schema`
(one source of per-column truth) and `constraints_display`
lists PK / NOT NULL / UNIQUE / DEFAULT.
No user-facing change yet — no grammar produces constrained
columns. Tests exercise creation, enforcement, describe, and
rebuild-preservation programmatically.
1177 tests pass (+5); clippy clean.
claude@clouddev1
a60e879f20