core/cgroup: fix TasksMaxScale percentage serialization (#41011)

bus_cgroup_set_tasks_max_scale() used a hand-rolled percentage format
that produced values ~10x too small (e.g., "TasksMax=4.0%" instead of
"TasksMax=40.00%").

On daemon-reload, the incorrect value was re-read, silently reducing
the effective TasksMax by ~10x and causing fork rejections on systems
with high thread counts.

Fix by using the existing PERMYRIAD macros, consistent with memory
property handlers (MemoryMax, MemoryHigh, MemoryLow, etc.).

Fixes: #41009

test/units/TEST-19-CGROUP.tasks-max-scale.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -ex
+set -o pipefail
+
+# shellcheck source=test/units/test-control.sh
+. "$(dirname "$0")"/test-control.sh
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
+testcase_tasks_max_scale_serialize() {
+    # Regression test for https://github.com/systemd/systemd/issues/41009
+    # TasksMaxScale was serialized as 4.0% instead of 40.00%, causing 10x reduction on daemon-reload
+
+    local unit="test-tasks-max.slice"
+    local unit_file="/run/systemd/system/${unit}"
+    local dropin_dir="/run/systemd/system.control/${unit}.d"
+
+    # shellcheck disable=SC2329
+    cleanup() (
+        set +e
+        rm -rf "${dropin_dir}"
+        rm -f "${unit_file}"
+        systemctl daemon-reload
+    )
+    trap cleanup RETURN
+
+    printf '[Slice]\n' >"${unit_file}"
+
+    systemctl daemon-reload
+
+    # Set TasksMax=40% via D-Bus — exercises bus_cgroup_set_tasks_max_scale()
+    systemctl set-property --runtime "${unit}" TasksMax=40%
+
+    # Verify drop-in file contains correct percentage (40.00%, not 4.0%)
+    grep -q '^TasksMax=40\.00%$' "${dropin_dir}/50-TasksMaxScale.conf"
+
+    # Capture value before daemon-reload
+    local tasks_max_before
+    tasks_max_before=$(systemctl show -P TasksMax "${unit}")
+
+    # Reload and verify value is preserved (the actual bug: value dropped 10x here)
+    systemctl daemon-reload
+
+    local tasks_max_after
+    tasks_max_after=$(systemctl show -P TasksMax "${unit}")
+    assert_eq "${tasks_max_before}" "${tasks_max_after}"
+}
+
+run_testcases