feat: accept \$var/Module.Assoc as widget DataSource (mendixlabs#198)

The grammar rejected association-path expressions as data sources for
DATAGRID/LISTVIEW/GALLERY, requiring the more verbose \`ASSOCIATION Path\`
keyword form. This added \`VARIABLE SLASH attributePathV3\` as an
alternative in \`dataSourceExprV3\` so the natural expression-like form
now works:

    DATAGRID items (DataSource: \$currentObject/Module.Order_OrderLine)

Both forms emit the same \"association\" data source type to the executor
and writer. Documentation updated in create-page.md and
master-detail-pages.md skills.

Closes mendixlabs#198.

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

Author: ako

.claude/skills/mendix/create-page.md

@@ -342,6 +342,8 @@ COLUMN colActions (Caption: 'Actions') {
 | `DataSource: $Variable` | Variable bound (requires DATAVIEW parent with entity) |
 | `DataSource: MICROFLOW Module.GetData()` | Microflow datasource |
 | `DataSource: SELECTION widgetName` | Listen to selection from another widget |
+| `DataSource: ASSOCIATION Path` | Retrieve by association from context (ByAssociation) |
+| `DataSource: $currentObject/Module.Assoc` | Sugar for `ASSOCIATION` — same semantics, reads more naturally |
 
 **With WHERE and SORT BY (inline in DataSource):**
 ```sql
@@ -804,7 +806,6 @@ The following features are NOT implemented in mxcli and require manual configura
 
 | Feature | Workaround |
 |---------|------------|
-| `DataSource: ASSOCIATION` | Use `DATABASE` with WHERE constraint, or microflow datasource |
 | Nested dataviews filtering by parent | Use microflow datasource or configure in Studio Pro |
 | Complex conditional visibility | Configure visibility rules in Studio Pro |
 | Widget-level security | Configure access rules in Studio Pro |

.claude/skills/mendix/master-detail-pages.md

@@ -86,7 +86,7 @@ LISTVIEW widgetName (DataSource: DATABASE Module.Entity, PageSize: 10) {
 
 Used inside the detail form to show related/associated data.
 
-**Note:** `DataSource: ASSOCIATION` is not yet fully implemented. Use `DATABASE` with a microflow datasource for related data.
+**Nested list by association:** Use `DataSource: $currentObject/Module.Assoc` (or the explicit `DataSource: ASSOCIATION Path` form) inside a parent DATAVIEW. Both forms produce the same BSON (ByAssociation data source). Example: `DATAGRID lines (DataSource: $currentObject/Order_OrderLine)` inside a `DATAVIEW dv (DataSource: DATABASE Order)`.
 
 ## Complete Example
 

mdl/grammar/MDLParser.g4

@@ -2168,13 +2168,14 @@ attributeListV3
 
 // V3 DataSource expressions
 dataSourceExprV3
-    : VARIABLE                                        // $ParamName
+    : VARIABLE SLASH attributePathV3                  // $currentObject/Module.Assoc (ByAssociation — sugar for ASSOCIATION)
+    | VARIABLE                                        // $ParamName
     | DATABASE FROM? qualifiedName                    // DATABASE [FROM] Entity [WHERE ...] [SORT BY ...]
       (WHERE (xpathConstraint (andOrXpath? xpathConstraint)* | expression))?
       (SORT_BY sortColumn (COMMA sortColumn)*)?
     | MICROFLOW qualifiedName microflowArgsV3?        // MICROFLOW Module.Flow
     | NANOFLOW qualifiedName microflowArgsV3?         // NANOFLOW Module.Flow
-    | ASSOCIATION attributePathV3                     // ASSOCIATION Path
+    | ASSOCIATION attributePathV3                     // ASSOCIATION Path (explicit form)
     | SELECTION IDENTIFIER                            // SELECTION widgetName
     ;