docs: add SET UNENFORCED PRIMARY KEY page and fix nav gaps

docs/src/operations/ddl/.pages

@@ -8,9 +8,11 @@ nav:
   - show-tables.md
   - describe-table.md
   - alter-table.md
+  - set-unenforced-primary-key.md
   - show-tblproperties.md
   - drop-table.md
   - create-index.md
+  - drop-index.md
   - show-indexes.md
   - create-branch.md
   - drop-branch.md

docs/src/operations/ddl/set-unenforced-primary-key.md

@@ -0,0 +1,56 @@
+# SET UNENFORCED PRIMARY KEY
+
+Declare primary key columns on a Lance table.
+
+!!! warning "Spark Extension Required"
+    This feature requires the Lance Spark SQL extension to be enabled. See [Spark SQL Extensions](../../config.md#spark-sql-extensions) for configuration details.
+
+## Overview
+
+The `SET UNENFORCED PRIMARY KEY` command records the primary key columns of a table as schema field metadata. Uniqueness is **not** enforced: no write path validates the declared columns, so duplicate values are accepted.
+
+## Syntax
+
+=== "SQL"
+    ```sql
+    ALTER TABLE <table> SET UNENFORCED PRIMARY KEY (<column> [, <column> ...]);
+    ```
+
+Column order is significant and is recorded alongside the declaration.
+
+## Examples
+
+### Single-column primary key
+
+=== "SQL"
+    ```sql
+    ALTER TABLE lance.db.users SET UNENFORCED PRIMARY KEY (id);
+    ```
+
+### Composite primary key
+
+=== "SQL"
+    ```sql
+    ALTER TABLE lance.db.users SET UNENFORCED PRIMARY KEY (id, name);
+    ```
+
+## Output
+
+The `SET UNENFORCED PRIMARY KEY` command returns the following information:
+
+| Column                | Type   | Description                                                  |
+|-----------------------|--------|--------------------------------------------------------------|
+| `status`              | String | The result status (`OK`).                                    |
+| `primary_key_columns` | String | The declared columns, comma-separated, in the order given.    |
+
+## How It Works
+
+The command commits a new table version that attaches `lance-schema:unenforced-primary-key` field metadata to each declared column, together with its one-based position in the key. Because the change is a normal commit, earlier versions keep their original schema and remain available for time travel.
+
+## Notes and Limitations
+
+- The declaration is **write-once**. Setting a primary key on a table that already has one fails; it cannot be changed or removed afterwards.
+- Columns must be non-nullable. Declare them `NOT NULL` when running [`CREATE TABLE`](create-table.md).
+- Columns must be primitive leaf fields. Struct and other nested columns are rejected.
+- The same column may not be listed twice.
+- Branch and tag identifiers are read-only, so the command is rejected against them.

docs/src/operations/dml/.pages

@@ -1,6 +1,7 @@
 title: DML
 nav:
   - insert-into.md
+  - insert-overwrite.md
   - update.md
   - delete.md
   - add-columns.md