feat: add ALTER ENTITY SET POSITION (x, y) for domain model layout

Allows repositioning entities on the domain model canvas without
recreating them:

  ALTER ENTITY Module.Customer SET POSITION (100, 200);

Wired through grammar, AST, visitor, and executor. Uses the existing
UpdateEntity writer method to update the Location field.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

.claude/skills/mendix/generate-domain-model.md

@@ -810,9 +810,13 @@ ALTER ENTITY Module.Customer
 -- Add an index
 ALTER ENTITY Module.Customer
   ADD INDEX idx_email (Email ASC);
+
+-- Reposition entity on domain model canvas
+ALTER ENTITY Module.Customer
+  SET POSITION (100, 200);
 ```
 
-**Supported operations:** ADD ATTRIBUTE, RENAME ATTRIBUTE, MODIFY ATTRIBUTE, DROP ATTRIBUTE, SET DOCUMENTATION, SET COMMENT, ADD INDEX, DROP INDEX.
+**Supported operations:** ADD ATTRIBUTE, RENAME ATTRIBUTE, MODIFY ATTRIBUTE, DROP ATTRIBUTE, SET DOCUMENTATION, SET COMMENT, ADD INDEX, DROP INDEX, SET POSITION.
 
 ### Entity Migration with CREATE OR MODIFY
 

docs-site/src/appendixes/quick-reference.md

@@ -47,6 +47,7 @@ Modifies an existing entity without full replacement.
 | Add index | `ALTER ENTITY Module.Name ADD INDEX (Col1 [ASC\|DESC], ...);` | |
 | Drop index | `ALTER ENTITY Module.Name DROP INDEX (Col1, ...);` | |
 | Set documentation | `ALTER ENTITY Module.Name SET DOCUMENTATION 'text';` | |
+| Set position | `ALTER ENTITY Module.Name SET POSITION (100, 200);` | Canvas position |
 
 **Example:**
 ```sql

docs-site/src/language/alter-entity.md

@@ -112,6 +112,9 @@ ALTER ENTITY <Module>.<Entity>
 
 ALTER ENTITY <Module>.<Entity>
   SET DOCUMENTATION '<text>'
+
+ALTER ENTITY <Module>.<Entity>
+  SET POSITION (<x>, <y>)
 ```
 
 ## See Also

docs-site/src/reference/domain-model/alter-entity.md

@@ -115,6 +115,15 @@ ALTER ENTITY Sales.Customer
     SET DOCUMENTATION 'Customer master data for the sales module.';
 ```
 
+### Set position
+
+Reposition an entity on the domain model canvas:
+
+```sql
+ALTER ENTITY Sales.Customer
+    SET POSITION (100, 200);
+```
+
 ## Notes
 
 - Each `ALTER ENTITY` statement performs a single operation. Chain multiple statements for multiple changes.

docs/01-project/MDL_QUICK_REFERENCE.md

@@ -49,6 +49,7 @@ Modifies an existing entity without full replacement.
 | Add index | `ALTER ENTITY Module.Name ADD INDEX (Col1 [ASC\|DESC], ...);` | |
 | Drop index | `ALTER ENTITY Module.Name DROP INDEX (Col1, ...);` | |
 | Set documentation | `ALTER ENTITY Module.Name SET DOCUMENTATION 'text';` | |
+| Set position | `ALTER ENTITY Module.Name SET POSITION (100, 200);` | Canvas position |
 
 **Example:**
 ```sql

docs/05-mdl-specification/01-language-reference.md

@@ -524,6 +524,9 @@ ALTER ENTITY <qualified-name>
 
 ALTER ENTITY <qualified-name>
   SET DOCUMENTATION '<text>'
+
+ALTER ENTITY <qualified-name>
+  SET POSITION (<x>, <y>)
 ```
 
 **Examples:**
@@ -547,6 +550,10 @@ ALTER ENTITY Sales.Customer
 -- Set documentation
 ALTER ENTITY Sales.Customer
   SET DOCUMENTATION 'Customer master data';
+
+-- Reposition entity on domain model canvas
+ALTER ENTITY Sales.Customer
+  SET POSITION (100, 200);
 ```
 
 ---

docs/06-mdl-reference/grammar-reference.md

@@ -2883,6 +2883,8 @@ alterEntityAction
     | | DROP COLUMN IDENTIFIER
     | | SET DOCUMENTATION STRING_LITERAL
     | | SET COMMENT STRING_LITERAL
+    | | SET STORE OWNER
+    | | SET POSITION LPAREN NUMBER_LITERAL COMMA NUMBER_LITERAL RPAREN
     | | ADD INDEX indexDefinition
     | | DROP INDEX IDENTIFIER
 ```

docs/11-proposals/PROPOSAL_entity_position.md

@@ -0,0 +1,111 @@
+# Proposal: Entity Positioning in ALTER ENTITY
+
+## Problem
+
+Entity positions in the domain model canvas can only be set during `CREATE ENTITY` via the `@position` annotation. There's no way to reposition existing entities without dropping and recreating them — which destroys associations, access rules, and references.
+
+This matters for:
+- **Scripts that create multiple related entities** — auto-positioning stacks them linearly, making the domain model hard to read in Studio Pro
+- **Domain model cleanup** — reorganizing entity layout after schema evolution
+- **Auto-layout** — enabling tools to compute optimal positions and apply them in a single pass
+
+## Proposed Syntax
+
+### Single entity
+
+```sql
+ALTER ENTITY Module.Customer SET POSITION (100, 200);
+```
+
+Consistent with existing `ALTER ENTITY ... SET DOCUMENTATION '...'` and `SET COMMENT '...'` patterns.
+
+### Multiple entities (batch repositioning)
+
+```sql
+ALTER ENTITY Module.Customer SET POSITION (100, 100);
+ALTER ENTITY Module.Order    SET POSITION (400, 100);
+ALTER ENTITY Module.Product  SET POSITION (400, 300);
+```
+
+### Auto-layout command (future extension)
+
+A separate top-level command for automatic layout of all entities in a module:
+
+```sql
+ARRANGE DOMAIN MODEL IN Module;
+```
+
+This is out of scope for the initial implementation but the grammar should not conflict with it.
+
+## Grammar Change
+
+Add one alternative to the `alterEntityAction` rule:
+
+```ebnf
+alterEntityAction
+    : ...existing alternatives...
+    | SET POSITION LPAREN NUMBER_LITERAL COMMA NUMBER_LITERAL RPAREN
+    ;
+```
+
+`POSITION` is already a keyword in the lexer (used by `@position` annotations and notebook actions).
+
+## AST Change
+
+Add a new operation constant and position field to `AlterEntityStmt`:
+
+```go
+const (
+    ...
+    AlterEntitySetPosition  // SET POSITION (x, y)
+)
+
+type AlterEntityStmt struct {
+    ...
+    Position  *Position  // For SET POSITION
+}
+```
+
+## Executor Change
+
+In `cmd_entities.go`, handle `AlterEntitySetPosition`:
+
+1. Find the entity in the domain model
+2. Update `entity.Location = model.Point{X: s.Position.X, Y: s.Position.Y}`
+3. Write the updated entity back via `writer.UpdateEntityLocation(entityID, location)`
+
+This is a lightweight operation — it only updates the `Location` field in the BSON, no structural changes.
+
+## DESCRIBE Output
+
+`DESCRIBE ENTITY` should include a `@position` annotation when the entity has a non-default position, so the output is round-trippable:
+
+```sql
+@position(100, 200)
+CREATE OR REPLACE PERSISTENT ENTITY Module.Customer (
+    Name: String(100),
+    ...
+);
+```
+
+Currently DESCRIBE omits the `@position` annotation. This should be added regardless of whether `ALTER ENTITY SET POSITION` is implemented, since it improves roundtrip fidelity.
+
+## Implementation Scope
+
+| Component | Change |
+|-----------|--------|
+| `MDLParser.g4` | Add `SET POSITION (x, y)` to `alterEntityAction` |
+| `MDLLexer.g4` | No change (`POSITION` already exists) |
+| `mdl/ast/ast_entity.go` | Add `AlterEntitySetPosition` op, `Position` field |
+| `mdl/visitor/visitor_entity.go` | Parse the new alternative |
+| `mdl/executor/cmd_entities.go` | Handle `AlterEntitySetPosition` |
+| `sdk/mpr/writer.go` | Add `UpdateEntityLocation()` (if not already present) |
+| DESCRIBE output | Add `@position` annotation |
+
+## Alternatives Considered
+
+**`@position` annotation on ALTER ENTITY** — e.g., `@position(100,200) ALTER ENTITY ...`. Rejected because annotations are a CREATE-time concept; SET is the established ALTER pattern.
+
+**Dedicated MOVE POSITION command** — e.g., `MOVE ENTITY Module.E TO POSITION (x, y)`. Rejected because MOVE already means "move to different module" in MDL.
+
+**ARRANGE command only** — Skip per-entity positioning, just auto-layout. Insufficient for scripts that need precise control (e.g., aligning entities in a specific pattern).

mdl-examples/doctype-tests/01-domain-model-examples.mdl

@@ -1445,6 +1445,10 @@ ALTER ENTITY DmTest.VATRate
 ALTER ENTITY DmTest.VATRate
   ADD INDEX (TaxRange ASC);
 
+-- Reposition entity on domain model canvas
+ALTER ENTITY DmTest.VATRate
+  SET POSITION (300, 400);
+
 -- ============================================================================
 -- PART 11: COMPLEX VIEW ENTITIES WITH SUBQUERIES
 -- ============================================================================

mdl/ast/ast_entity.go

@@ -66,6 +66,7 @@ const (
 	AlterEntityAddIndex                              // ADD INDEX
 	AlterEntityDropIndex                             // DROP INDEX
 	AlterEntitySetStoreOwner                         // SET STORE OWNER
+	AlterEntitySetPosition                           // SET POSITION (x, y)
 )
 
 // AlterEntityStmt represents: ALTER ENTITY Module.Name ADD/DROP/RENAME/MODIFY ATTRIBUTE ...
@@ -82,6 +83,7 @@ type AlterEntityStmt struct {
 	Comment       string     // For SET COMMENT
 	Index         *Index     // For ADD INDEX
 	IndexName     string     // For DROP INDEX
+	Position      *Position  // For SET POSITION
 }
 
 func (s *AlterEntityStmt) isStatement() {}