test: expand coverage across all layers -- 1,268 new tests, fix replace-color div-by-zero

14-agent parallel test expansion covering integration, unit, E2E, E2E-Docker,
cross-format matrix, adversarial, GUI navigation/tools/settings/visual/a11y/perf.

- Integration: expand 23 tool test files with HEIC, stress, batch, edge cases
- Unit: close coverage gaps in image-engine, stores, lib (metadata, auto-enhance,
  connection-store, lazy-with-retry, collage/file-store HEIC preview)
- AI bridge: 141 new tests for dispatcher buffering, crash recovery, OOM/segfault
- Cross-format: 794 parameterized tests (16 formats x 12 tools + no-crash matrix)
- Adversarial: memory stress (50x large file), zero-byte, corrupted headers, unicode
- E2E-Docker: expand 8 spec files with dimension verification, pipeline chains
- GUI E2E: tool UI settings/interactions for all 47 tools, remove all test.skip,
  RBAC per-role verification, visual screenshot naming, cross-browser smoke tests,
  a11y ARIA/focus/contrast, performance budgets, 15-tool stability test
- Fix: replace-color.ts tolerance=0 caused division-by-zero producing NaN pixels

Total: 8,958 tests passing across 202 files. Zero failures, zero skips.

tests/unit/ai/face-landmarks.test.ts

@@ -263,4 +263,41 @@ describe("detectFaceLandmarks", () => {
       expect(options.onProgress).toBeUndefined();
     });
   });
+
+  describe("edge cases", () => {
+    it("propagates writeFile error", async () => {
+      vi.mocked(writeFile).mockRejectedValueOnce(new Error("Permission denied"));
+
+      await expect(detectFaceLandmarks(FAKE_INPUT)).rejects.toThrow("Permission denied");
+    });
+
+    it("propagates segfault from bridge", async () => {
+      vi.mocked(runPythonWithProgress).mockRejectedValue(
+        new Error("Process crashed (segmentation fault)"),
+      );
+
+      await expect(detectFaceLandmarks(FAKE_INPUT)).rejects.toThrow("segmentation fault");
+    });
+
+    it("uses timestamps in temp file names", async () => {
+      await detectFaceLandmarks(FAKE_INPUT);
+
+      const writePath = vi.mocked(writeFile).mock.calls[0][0] as string;
+      // File path includes a numeric timestamp
+      expect(writePath).toMatch(/face_landmarks_\d+\.png$/);
+    });
+
+    it("handles faceDetected true with missing landmarks", async () => {
+      vi.mocked(parseStdoutJson).mockReturnValue({
+        success: true,
+        faceDetected: true,
+        imageWidth: 1920,
+        imageHeight: 1080,
+      });
+
+      const result = await detectFaceLandmarks(FAKE_INPUT);
+      expect(result.faceDetected).toBe(true);
+      expect(result.landmarks).toBeNull();
+    });
+  });
 });