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/image-engine/operations.test.ts

@@ -764,6 +764,13 @@ describe("compress", () => {
     expect(Math.abs(buf.length - targetBytes) / targetBytes).toBeLessThan(0.5);
   });
 
+  it("compresses with format=avif adds effort option", async () => {
+    const img = sharp(png200x150);
+    const result = await compress(img, { quality: 50, format: "avif" });
+    const meta = await getMeta(result);
+    expect(meta.format).toBe("heif");
+  });
+
   it("target size falls back to bestQuality=1 when bestBuffer stays null", async () => {
     // Use a very tiny target that forces all iterations to overshoot,
     // so bestBuffer never gets assigned and the fallback path runs