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/replace-color.test.ts

@@ -563,3 +563,106 @@ describe("SVG input", () => {
     expect(result.downloadUrl).toBeDefined();
   });
 });
+
+// ── makeTransparent explicitly false ──────────────────────────
+describe("Explicit makeTransparent false", () => {
+  it("replaces color normally when makeTransparent is false", async () => {
+    const res = await postTool(
+      { sourceColor: "#FF0000", targetColor: "#00FF00", makeTransparent: false, tolerance: 30 },
+      solidRedBuffer,
+    );
+    expect(res.statusCode).toBe(200);
+    const result = JSON.parse(res.body);
+    expect(result.downloadUrl).toBeDefined();
+
+    // Download and verify color changed (not transparent)
+    const dlRes = await app.inject({
+      method: "GET",
+      url: result.downloadUrl,
+      headers: { authorization: `Bearer ${adminToken}` },
+    });
+    const { data } = await sharp(dlRes.rawPayload)
+      .removeAlpha()
+      .raw()
+      .toBuffer({ resolveWithObject: true });
+    // First pixel should be green-ish
+    expect(data[1]).toBeGreaterThan(data[0]); // green > red
+  });
+});
+
+// ── makeTransparent on HEIC input ─────────────────────────────
+describe("makeTransparent on HEIC", () => {
+  it("makes pixels transparent in a HEIC image", { timeout: 120_000 }, async () => {
+    const HEIC = readFileSync(join(FIXTURES, "test-200x150.heic"));
+    const res = await postTool(
+      { sourceColor: "#808080", makeTransparent: true, tolerance: 100 },
+      HEIC,
+      "photo.heic",
+      "image/heic",
+    );
+    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.channels).toBe(4);
+  });
+});
+
+// ── Invalid settings JSON ─────────────────────────────────────
+describe("Invalid settings JSON", () => {
+  it("rejects malformed settings JSON", 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/replace-color",
+      payload,
+      headers: {
+        "content-type": contentType,
+        authorization: `Bearer ${adminToken}`,
+      },
+    });
+    expect(res.statusCode).toBe(400);
+  });
+});
+
+// ── Verify pixel accuracy with exact tolerance ────────────────
+describe("Pixel accuracy verification", () => {
+  it("replaces all pixels in solid image at tolerance=0", async () => {
+    const res = await postTool(
+      { sourceColor: "#FF0000", targetColor: "#FFFFFF", tolerance: 0 },
+      solidRedBuffer,
+    );
+    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}` },
+    });
+
+    // All pixels should be white now
+    const { data, info } = await sharp(dlRes.rawPayload)
+      .removeAlpha()
+      .raw()
+      .toBuffer({ resolveWithObject: true });
+
+    // Check a sample of pixels are white (R=255, G=255, B=255)
+    let whiteCount = 0;
+    const totalPixels = info.width * info.height;
+    for (let i = 0; i < data.length; i += 3) {
+      if (data[i] > 250 && data[i + 1] > 250 && data[i + 2] > 250) {
+        whiteCount++;
+      }
+    }
+    // Nearly all pixels should be white
+    expect(whiteCount / totalPixels).toBeGreaterThan(0.9);
+  });
+});