perf(web): reuse incremental build and status caches

tools/web/build-web-engine-variants.sh

@@ -8,9 +8,18 @@ configure_variant() {
   local name="$1"
   local threads="$2"
   local build_dir="${repo_root}/build_web-${name}"
-  emcmake cmake --fresh -S "${repo_root}/web/engine" -B "${build_dir}" -G Ninja \
-    -DCMAKE_BUILD_TYPE=Release -DWEB_ENGINE_THREADS="${threads}"
-  cmake --build "${build_dir}" --target web_engine
+  local configure_args=(
+    -S "${repo_root}/web/engine"
+    -B "${build_dir}"
+    -G Ninja
+    -DCMAKE_BUILD_TYPE=Release
+    -DWEB_ENGINE_THREADS="${threads}"
+  )
+  if [[ "${WEB_ENGINE_FORCE_RECONFIGURE:-0}" == "1" ]]; then
+    configure_args=(--fresh "${configure_args[@]}")
+  fi
+  emcmake cmake "${configure_args[@]}"
+  cmake --build "${build_dir}" --target web_engine --parallel "${WEB_BUILD_JOBS:-$(bash "${repo_root}/tools/web/compute-build-jobs.sh")}"
   test -s "${build_dir}/web_engine.js"
   test -s "${build_dir}/web_engine.wasm"
   printf '%s variant ok: %s\n' "${name}" "${build_dir}"