test(18-04): add failing regression test for campaign report_count double-increment

- Sibling report already in campaign-1 re-matches via Tier 3; asserts zero
  UPDATE campaigns / INSERT campaigns / UPDATE reports calls (CAMP-02/CR-01)

lib/services/campaign-grouping-service.test.ts

@@ -333,4 +333,31 @@ describe('groupReportIntoCampaign', () => {
     // a second time for this report re-run.
     expect(callsContaining('UPDATE campaigns')).toHaveLength(0);
   });
+
+  it('re-analyzing a report already linked to a campaign that a sibling report re-matches does not increment report_count a second time', async () => {
+    // Own report is already linked to campaign-1. A SIBLING report (not this
+    // report's own excluded row) also belongs to campaign-1 and matches via
+    // Tier 3 (sender + subject + client). This is the CAMP-02/CR-01 gap:
+    // tier queries only exclude the report's OWN row, not siblings already
+    // in the same campaign, so a naive re-match would double-increment.
+    stage({
+      ownReport: [{ ...REPORT_ROW, campaign_id: 'campaign-1' }],
+      ownMessage: [],
+      tier3: [{ campaign_id: 'campaign-1', title: 'Invoice Alert' }],
+    });
+
+    const result = await groupReportIntoCampaign('report-self');
+
+    expect(result).toEqual({
+      campaignId: 'campaign-1',
+      groupMethod: 'sender_subject_client',
+      created: false,
+    });
+
+    // Critical assertions: no double-increment, no phantom new campaign, no
+    // redundant re-link.
+    expect(callsContaining('UPDATE campaigns')).toHaveLength(0);
+    expect(callsContaining('INSERT INTO campaigns')).toHaveLength(0);
+    expect(callsContaining('UPDATE reports SET campaign_id')).toHaveLength(0);
+  });
 });