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/colorization.test.ts

@@ -198,5 +198,48 @@ describe("colorize", () => {
         expect.objectContaining({ onProgress }),
       );
     });
+
+    it("omits onProgress when not provided", async () => {
+      await colorize(FAKE_INPUT, FAKE_OUTPUT_DIR);
+
+      const options = vi.mocked(runPythonWithProgress).mock.calls[0][2];
+      expect(options.onProgress).toBeUndefined();
+    });
+  });
+
+  describe("sharp conversion errors", () => {
+    it("propagates sharp toBuffer error", async () => {
+      vi.mocked(sharp).mockImplementation(
+        () =>
+          ({
+            png: vi.fn().mockReturnThis(),
+            toBuffer: vi.fn().mockRejectedValue(new Error("Corrupt image data")),
+          }) as unknown as ReturnType<typeof sharp>,
+      );
+
+      await expect(colorize(FAKE_INPUT, FAKE_OUTPUT_DIR)).rejects.toThrow("Corrupt image data");
+    });
+  });
+
+  describe("edge cases", () => {
+    it("passes empty options as empty JSON object", async () => {
+      await colorize(FAKE_INPUT, FAKE_OUTPUT_DIR, {});
+
+      const args = vi.mocked(runPythonWithProgress).mock.calls[0][1];
+      expect(JSON.parse(args[2])).toEqual({});
+    });
+
+    it("handles zero width and height in response", async () => {
+      vi.mocked(parseStdoutJson).mockReturnValue({
+        success: true,
+        width: 0,
+        height: 0,
+        method: "deoldify",
+      });
+
+      const result = await colorize(FAKE_INPUT, FAKE_OUTPUT_DIR);
+      expect(result.width).toBe(0);
+      expect(result.height).toBe(0);
+    });
   });
 });