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/integration/sharpening.test.ts

@@ -655,3 +655,105 @@ describe("Output format preservation", () => {
     expect(meta.height).toBe(50);
   });
 });
+
+// ── High-pass with default kernelSize 3 ──────────────────────
+describe("High-pass default kernelSize", () => {
+  it("uses default kernelSize (3) when not specified", async () => {
+    const res = await postTool({ method: "high-pass", strength: 50 });
+    expect(res.statusCode).toBe(200);
+    const result = JSON.parse(res.body);
+    expect(result.downloadUrl).toBeDefined();
+    expect(result.processedSize).toBeGreaterThan(0);
+  });
+});
+
+// ── Before-after comparison for each method ────────────────────
+describe("Before-after pixel comparison", () => {
+  it("unsharp-mask changes pixel data", async () => {
+    const res = await postTool({ method: "unsharp-mask", amount: 500, radius: 2.0, threshold: 0 });
+    expect(res.statusCode).toBe(200);
+    const result = JSON.parse(res.body);
+
+    const dlRes = await app.inject({
+      method: "GET",
+      url: result.downloadUrl,
+      headers: { authorization: `Bearer ${adminToken}` },
+    });
+
+    expect(Buffer.compare(dlRes.rawPayload, PNG)).not.toBe(0);
+  });
+
+  it("high-pass changes pixel data", async () => {
+    const res = await postTool({ method: "high-pass", strength: 80, kernelSize: 5 });
+    expect(res.statusCode).toBe(200);
+    const result = JSON.parse(res.body);
+
+    const dlRes = await app.inject({
+      method: "GET",
+      url: result.downloadUrl,
+      headers: { authorization: `Bearer ${adminToken}` },
+    });
+
+    expect(Buffer.compare(dlRes.rawPayload, PNG)).not.toBe(0);
+  });
+});
+
+// ── Invalid settings JSON ────────────────────────────────────
+describe("Invalid settings JSON", () => {
+  it("rejects malformed settings JSON string", async () => {
+    const { body: payload, contentType } = createMultipartPayload([
+      { name: "file", filename: "test.png", contentType: "image/png", content: PNG },
+      { name: "settings", content: "not-valid-json" },
+    ]);
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/sharpening",
+      payload,
+      headers: {
+        "content-type": contentType,
+        authorization: `Bearer ${adminToken}`,
+      },
+    });
+    expect(res.statusCode).toBe(400);
+  });
+});
+
+// ── Extreme sharpening on tiny image ──────────────────────────
+describe("Extreme sharpening on tiny image", () => {
+  it("applies maximum sharpening to 1x1 pixel image", async () => {
+    const TINY = readFileSync(join(FIXTURES, "test-1x1.png"));
+    const res = await postTool(
+      { method: "unsharp-mask", amount: 1000, radius: 5.0, threshold: 0 },
+      TINY,
+      "tiny.png",
+      "image/png",
+    );
+    expect(res.statusCode).toBe(200);
+    const result = JSON.parse(res.body);
+    expect(result.processedSize).toBeGreaterThan(0);
+  });
+});
+
+// ── Adaptive with denoise on JPEG ─────────────────────────────
+describe("Adaptive with denoise on JPEG", () => {
+  it("applies adaptive sharpening with medium denoise on JPEG", async () => {
+    const res = await postTool(
+      { method: "adaptive", sigma: 2.0, denoise: "medium" },
+      JPG,
+      "test.jpg",
+      "image/jpeg",
+    );
+    expect(res.statusCode).toBe(200);
+    const result = JSON.parse(res.body);
+
+    const dlRes = await app.inject({
+      method: "GET",
+      url: result.downloadUrl,
+      headers: { authorization: `Bearer ${adminToken}` },
+    });
+    const meta = await sharp(dlRes.rawPayload).metadata();
+    expect(meta.format).toBe("jpeg");
+    expect(meta.width).toBe(100);
+    expect(meta.height).toBe(100);
+  });
+});