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/inpainting.test.ts

@@ -199,4 +199,51 @@ describe("inpaint", () => {
       expect(options.onProgress).toBeUndefined();
     });
   });
+
+  describe("sharp conversion errors", () => {
+    it("propagates sharp error on mask conversion", async () => {
+      let callCount = 0;
+      vi.mocked(sharp).mockImplementation(() => {
+        callCount++;
+        if (callCount === 1) {
+          return {
+            png: vi.fn().mockReturnThis(),
+            toBuffer: vi.fn().mockResolvedValue(Buffer.from("mock-png-data")),
+          } as unknown as ReturnType<typeof sharp>;
+        }
+        return {
+          png: vi.fn().mockReturnThis(),
+          toBuffer: vi.fn().mockRejectedValue(new Error("Corrupt mask")),
+        } as unknown as ReturnType<typeof sharp>;
+      });
+
+      await expect(inpaint(FAKE_INPUT, FAKE_MASK, FAKE_OUTPUT_DIR)).rejects.toThrow("Corrupt mask");
+    });
+  });
+
+  describe("edge cases", () => {
+    it("propagates segfault from bridge", async () => {
+      vi.mocked(runPythonWithProgress).mockRejectedValue(
+        new Error("Process crashed (segmentation fault)"),
+      );
+
+      await expect(inpaint(FAKE_INPUT, FAKE_MASK, FAKE_OUTPUT_DIR)).rejects.toThrow(
+        "segmentation fault",
+      );
+    });
+
+    it("propagates readFile error after successful Python run", async () => {
+      vi.mocked(readFile).mockRejectedValueOnce(new Error("ENOENT: output missing"));
+
+      await expect(inpaint(FAKE_INPUT, FAKE_MASK, FAKE_OUTPUT_DIR)).rejects.toThrow(
+        "output missing",
+      );
+    });
+
+    it("propagates writeFile error when writing input", async () => {
+      vi.mocked(writeFile).mockRejectedValueOnce(new Error("ENOSPC: disk full"));
+
+      await expect(inpaint(FAKE_INPUT, FAKE_MASK, FAKE_OUTPUT_DIR)).rejects.toThrow("disk full");
+    });
+  });
 });