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.

apps/api/src/routes/tools/replace-color.ts

@@ -66,7 +66,7 @@ export function registerReplaceColor(app: FastifyInstance) {
             pixels[i + 3] = 0; // Make transparent
           } else {
             // Blend: closer to source color = more of target color
-            const blend = 1 - dist / maxDist;
+            const blend = maxDist > 0 ? 1 - dist / maxDist : 1;
             pixels[i] = Math.round(pixels[i] * (1 - blend) + target.r * blend);
             pixels[i + 1] = Math.round(pixels[i + 1] * (1 - blend) + target.g * blend);
             pixels[i + 2] = Math.round(pixels[i + 2] * (1 - blend) + target.b * blend);