db: column-constraint infrastructure — NOT NULL / UNIQUE / DEFAULT (ADR-0029)

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.

src/output_render.rs

@@ -354,12 +354,20 @@ fn type_display(c: &ColumnDescription) -> String {
 }
 
 fn constraints_display(c: &ColumnDescription) -> String {
-    let mut parts: Vec<&str> = Vec::new();
+    let mut parts: Vec<String> = Vec::new();
     if c.primary_key {
-        parts.push("PK");
+        parts.push("PK".to_string());
     }
     if c.notnull {
-        parts.push("NOT NULL");
+        parts.push("NOT NULL".to_string());
+    }
+    // ADR-0029: a PK column's implicit uniqueness is already
+    // conveyed by `PK`; `unique` is only set for non-PK columns.
+    if c.unique {
+        parts.push("UNIQUE".to_string());
+    }
+    if let Some(default) = &c.default {
+        parts.push(format!("DEFAULT {default}"));
     }
     parts.join(", ")
 }
@@ -511,6 +519,8 @@ mod tests {
             },
             notnull,
             primary_key: pk,
+            unique: false,
+            default: None,
         }
     }
 
@@ -978,6 +988,8 @@ mod tests {
                 sqlite_type: "INTEGER".to_string(),
                 notnull: false,
                 primary_key: false,
+                unique: false,
+                default: None,
             }],
             outbound_relationships: Vec::new(),
             inbound_relationships: Vec::new(),