docs(website): fill the six Reference stubs with worked examples + output

Expand columns, relationships, indexes, constraints, inserting-and-editing-
data, and querying-and-inspecting from syntax-only stubs into full pages,
each with worked examples on the library schema and real captured app output
(structure boxes, relationship diagrams, data tables, show-lists, query
plans, cascade summaries). All output captured verbatim from the app — never
hand-drawn. Both simple- and advanced-mode forms shown where both apply;
advanced syntax verified against tests/source.

STYLE.md: record the output-block convention (plain unlabelled fence,
captured from a throwaway harness, not hand-drawn).

Verified: pnpm build clean (24 pages); no forbidden terms; internal links
and heading anchors resolve.

website/src/content/docs/reference/constraints.md

@@ -1,17 +1,99 @@
 ---
 title: Constraints
-description: NOT NULL, UNIQUE, DEFAULT, and CHECK constraints on columns.
+description: NOT NULL, UNIQUE, DEFAULT, and CHECK constraints that keep data valid.
 sidebar:
   order: 6
 ---
 
-:::note[In progress]
-The syntax below is complete and accurate; worked examples on the library
-schema are being added.
-:::
+Constraints are rules the database enforces on a column's values, so invalid
+data can never be stored in the first place. There are four:
 
-Constraints keep your data valid. Declare them in the column spec at
-`create table` / `add column` time, or change them on an existing column:
+| Constraint | Rule |
+|---|---|
+| `not null` | The column must always have a value. |
+| `unique` | No two rows may share a value (empty values aside). |
+| `default <value>` | The value to use when an `insert` omits the column. |
+| `check (<expr>)` | Every value must satisfy a condition you write. |
+
+## Adding constraints
+
+You can add a constraint to an existing column at any time. In the
+[example library](/getting-started/example-library/), a book's publication year
+should be positive, and a sensible fallback is useful when it is unknown:
+
+```rdbms
+add constraint default 2000 to books.published
+add constraint check (published > 1400) to books.published
+```
+
+Constraints show up in the column's **Constraints** cell in `show table`:
+
+```rdbms
+show table books
+```
+
+```
+books
+┌───────────┬────────┬──────────────────────────────────────────┐
+│ Name      │ Type   │ Constraints                              │
+├───────────┼────────┼──────────────────────────────────────────┤
+│ book_id   │ serial │ PK                                       │
+│ title     │ text   │ NOT NULL                                 │
+│ author_id │ int    │                                          │
+│ published │ int    │ DEFAULT 2000, CHECK ("published" > 1400) │
+│ isbn      │ text   │ UNIQUE                                   │
+└───────────┴────────┴──────────────────────────────────────────┘
+```
+
+Here `title` is `NOT NULL` and `isbn` is `UNIQUE` — both were declared when the
+table was built (see below). `PK` marks the primary key, which is fixed at
+creation time ([Tables](/reference/tables/)).
+
+## Declaring constraints when you build the table
+
+Constraints can also be part of a column's spec from the start. In advanced
+mode they sit inline in `CREATE TABLE`:
+
+```sql
+create table books (
+  book_id serial primary key,
+  title text not null,
+  author_id int,
+  published int default 2000 check (published > 1400),
+  isbn text unique
+)
+```
+
+## Adding a constraint to existing data
+
+When you add a constraint to a column that already holds rows, the playground
+checks the current data first. If any row would break the new rule — a `null`
+where you are adding `not null`, a duplicate where you are adding `unique`, or a
+value that fails a `check` — the change is **refused** and the offending rows
+are listed, so you can fix the data and try again. Nothing is changed until the
+constraint can hold for every row.
+
+## Removing a constraint
+
+Name the kind to drop it (there is at most one of each kind per column):
+
+```rdbms
+drop constraint check from books.published
+drop constraint default from books.published
+```
+
+## In advanced mode
+
+A table-level `check` or `unique` can be added and dropped with `ALTER TABLE`;
+a named constraint can then be dropped by that name:
+
+```sql
+alter table books add constraint published_range check (published > 1400)
+alter table books add unique (title, author_id)
+alter table books drop constraint published_range
+```
+
+## Syntax
 
 ```rdbms-syntax
 add constraint not null to <Table>.<col>
@@ -21,10 +103,5 @@ add constraint check (<expr>) to <Table>.<col>
 drop constraint (not null | unique | default | check) from <Table>.<col>
 ```
 
-Adding a constraint to a column that already has rows is checked first — if
-existing data would violate it, the change is refused and the offending rows
-are shown.
-
-In advanced mode these are the standard column- and table-level constraints
-in `CREATE TABLE` / `ALTER TABLE`. The primary key is fixed at creation (see
-[Tables](/reference/tables/)).
+See also [Columns](/reference/columns/), [Types](/reference/types/), and
+[Inserting & editing data](/reference/inserting-and-editing-data/).