fix: reject startAfter on multi-source (join) effects

Cursor gate state is global to the effect, so a cursor from one source
collection could incorrectly open the gate for another. Validate at
construction time that startAfter is only used with single-source
effects until per-source cursor tracking is implemented.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: KyleAMathews

packages/db/src/query/effect.ts

@@ -410,6 +410,16 @@ class EffectPipelineRunner<TRow extends object, TKey extends string | number> {
     this.collections = extractCollectionsFromQuery(this.query)
     const aliasesById = extractCollectionAliases(this.query)
 
+    if (
+      config.startAfter !== undefined &&
+      Object.keys(this.collections).length > 1
+    ) {
+      throw new Error(
+        `startAfter is only supported for single-source effects. ` +
+          `This effect queries ${Object.keys(this.collections).length} collections.`,
+      )
+    }
+
     // Build alias → collection map
     this.collectionByAlias = {}
     for (const [collectionId, aliases] of aliasesById.entries()) {

packages/db/tests/effect.test.ts

@@ -568,6 +568,24 @@ describe(`createEffect`, () => {
 
       await effect.dispose()
     })
+
+    it(`should throw when startAfter is used with multi-source effects`, () => {
+      const users = createUsersCollection()
+      const issues = createIssuesCollection()
+
+      expect(() => {
+        createEffect({
+          query: (q) =>
+            q
+              .from({ issue: issues })
+              .join({ user: users }, ({ issue, user }) =>
+                eq(issue.userId, user.id),
+              ),
+          onBatch: () => {},
+          startAfter: 1,
+        })
+      }).toThrow(/single-source/)
+    })
   })
 
   describe(`filtered queries`, () => {