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/watermark-image.test.ts

@@ -687,4 +687,70 @@ describe("watermark-image", () => {
     const json = JSON.parse(res.body);
     expect(json.downloadUrl).toBeDefined();
   });
+
+  // ── Rejects invalid position value ──────────────────────────────
+
+  it("rejects invalid position value (tiled not supported)", async () => {
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "main.png", contentType: "image/png", content: PNG },
+      { name: "watermark", filename: "wm.png", contentType: "image/png", content: SMALL_PNG },
+      { name: "settings", content: JSON.stringify({ position: "tiled" }) },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/watermark-image",
+      headers: { authorization: `Bearer ${adminToken}`, "content-type": contentType },
+      body,
+    });
+
+    expect(res.statusCode).toBe(400);
+  });
+
+  // ── SVG watermark image ─────────────────────────────────────────
+
+  it("processes SVG watermark image", async () => {
+    const SVG = readFileSync(join(FIXTURES, "test-100x100.svg"));
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "main.png", contentType: "image/png", content: PNG },
+      { name: "watermark", filename: "wm.svg", contentType: "image/svg+xml", content: SVG },
+      { name: "settings", content: JSON.stringify({ scale: 20, position: "center" }) },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/watermark-image",
+      headers: { authorization: `Bearer ${adminToken}`, "content-type": contentType },
+      body,
+    });
+
+    expect(res.statusCode).toBe(200);
+    const json = JSON.parse(res.body);
+    expect(json.downloadUrl).toBeDefined();
+  });
+
+  // ── Opacity at boundaries with scale variations ─────────────────
+
+  it("applies opacity=50 with scale=75 at top-right position", async () => {
+    const { body, contentType } = createMultipartPayload([
+      { name: "file", filename: "main.png", contentType: "image/png", content: PNG },
+      { name: "watermark", filename: "wm.png", contentType: "image/png", content: SMALL_PNG },
+      {
+        name: "settings",
+        content: JSON.stringify({ opacity: 50, scale: 75, position: "top-right" }),
+      },
+    ]);
+
+    const res = await app.inject({
+      method: "POST",
+      url: "/api/v1/tools/watermark-image",
+      headers: { authorization: `Bearer ${adminToken}`, "content-type": contentType },
+      body,
+    });
+
+    expect(res.statusCode).toBe(200);
+    const json = JSON.parse(res.body);
+    expect(json.downloadUrl).toBeDefined();
+    expect(json.processedSize).toBeGreaterThan(0);
+  });
 });