feat(ddl): record schema version metadata on ADD/DROP COLUMN

ALTER TABLE ... ADD COLUMN now stamps the new column's `added_at_version`
with the bumped schema version before appending it to the live column
list, so the read path can distinguish columns that did not exist when
older tuples were written.

ALTER TABLE ... DROP COLUMN now records a `DroppedColumn` tombstone
(definition, original position, version at drop) instead of silently
discarding the column definition. This allows the reader to reconstruct
the physical layout of any tuple written before the drop without
requiring row migration.

The CONVERT path initialises `dropped_columns` to an empty vec to keep
all `StrictSchema` construction sites consistent.

Author: farhan-syah

nodedb/src/control/server/pgwire/ddl/collection/alter/add_column.rs

@@ -62,8 +62,11 @@ pub async fn alter_table_add_column(
                             &format!("column '{}' already exists", column.name),
                         ));
                     }
+                    let new_version = schema.version.saturating_add(1);
+                    let mut column = column;
+                    column.added_at_version = new_version;
                     schema.columns.push(column);
-                    schema.version = schema.version.saturating_add(1);
+                    schema.version = new_version;
 
                     let mut updated = coll;
                     updated.collection_type = nodedb_types::CollectionType::strict(schema.clone());

nodedb/src/control/server/pgwire/ddl/collection/alter/drop_column.rs

@@ -79,8 +79,16 @@ pub async fn alter_collection_drop_column(
         ));
     }
 
-    schema.columns.remove(idx);
-    schema.version = schema.version.saturating_add(1);
+    let dropped_def = schema.columns.remove(idx);
+    let new_version = schema.version.saturating_add(1);
+    schema
+        .dropped_columns
+        .push(nodedb_types::columnar::DroppedColumn {
+            def: dropped_def,
+            position: idx,
+            dropped_at_version: new_version,
+        });
+    schema.version = new_version;
 
     let mut updated = coll;
     updated.collection_type = nodedb_types::CollectionType::strict(schema.clone());

nodedb/src/control/server/pgwire/ddl/convert.rs

@@ -92,6 +92,7 @@ pub async fn convert_collection(
             let schema = nodedb_types::columnar::StrictSchema {
                 columns,
                 version: 1,
+                dropped_columns: Vec::new(),
             };
             if target_type == "kv" {
                 nodedb_types::CollectionType::kv(schema)