B2/C2: column drop / rename / change-type DSL commands

Closes B2 (rebuild-table reused outside relationships) and
C2 (full add/drop/rename/change-type column operations).

* drop column [from] [table] <T>: <col>
  - ALTER TABLE DROP COLUMN (SQLite 3.35+) + metadata
    cleanup in __rdbms_playground_columns.
  - Refuses PK columns and columns involved in a declared
    relationship (drop the relationship first).

* rename column [in] [table] <T>: <old> to <new>
  - ALTER TABLE RENAME COLUMN (SQLite 3.25+); SQLite
    cascades the rename through FK declarations on other
    tables.
  - Mirrors the new name into both metadata tables
    (__rdbms_playground_columns, __rdbms_playground_relationships)
    so describes stay accurate after a rename.
  - Refuses identity rename and name collisions.

* change column [in] [table] <T>: <col> (<newtype>)
  - Routes through the rebuild_table primitive (ADR-0013)
    since SQLite ALTER doesn't support type changes.
    INSERT INTO new SELECT FROM old; STRICT typing enforces
    cell compatibility, transaction rolls back on mismatch.
  - Refuses PK columns, relationship-involved columns,
    `serial` target, and no-op same-type changes.

Adds 20 tests (parser + db layer); updates the in-app help
listing. Both prepositions independently optional in each
new command, matching `add column`'s grammar shape.

Total: 449 passing, 0 failing, 0 skipped (up from 429).
Clippy clean.

Known spec gap: column-type-change conversion compatibility
is not yet documented (currently relies on SQLite STRICT
errors); follow-up will close this.

src/dsl/command.rs

@@ -44,6 +44,35 @@ pub enum Command {
         column: String,
         ty: Type,
     },
+    /// Remove a column from a table. Refused if the column is
+    /// part of the primary key or is involved in a declared
+    /// relationship — drop the relationship first.
+    DropColumn {
+        table: String,
+        column: String,
+    },
+    /// Rename a column. SQLite handles cascading renames in
+    /// FK references on other tables; the executor mirrors
+    /// the change into our `__rdbms_playground_columns` and
+    /// `__rdbms_playground_relationships` metadata tables in
+    /// the same transaction.
+    RenameColumn {
+        table: String,
+        old: String,
+        new: String,
+    },
+    /// Change a column's type. Implemented via the
+    /// rebuild-table primitive (ADR-0013) since SQLite's
+    /// ALTER TABLE does not support type changes. Refused
+    /// for PK columns and for columns involved in a declared
+    /// relationship — those would require cascading FK
+    /// type updates the v1 surface deliberately doesn't
+    /// expose.
+    ChangeColumnType {
+        table: String,
+        column: String,
+        ty: Type,
+    },
     /// Establish a 1:n relationship: parent_table.parent_column
     /// is the primary-key side; child_table.child_column is the
     /// foreign-key side. `name` is optional — when `None`, the
@@ -144,6 +173,9 @@ impl Command {
             Self::CreateTable { .. } => "create table",
             Self::DropTable { .. } => "drop table",
             Self::AddColumn { .. } => "add column",
+            Self::DropColumn { .. } => "drop column",
+            Self::RenameColumn { .. } => "rename column",
+            Self::ChangeColumnType { .. } => "change column",
             Self::AddRelationship { .. } => "add relationship",
             Self::DropRelationship { .. } => "drop relationship",
             Self::ShowTable { .. } => "show table",
@@ -166,6 +198,9 @@ impl Command {
             | Self::ShowTable { name }
             | Self::ShowData { name } => name,
             Self::AddColumn { table, .. }
+            | Self::DropColumn { table, .. }
+            | Self::RenameColumn { table, .. }
+            | Self::ChangeColumnType { table, .. }
             | Self::Insert { table, .. }
             | Self::Update { table, .. }
             | Self::Delete { table, .. } => table,