Missing serial lock in acpi_panasonic_shutdown triggers KKASSERT panic on INVARIANTS kernels

Summary

acpi_panasonic_shutdown calls hkey_sound_mute without ACPI_SERIAL_BEGIN(panasonic). hkey_sound_mute asserts lock via ACPI_SERIAL_ASSERT(:374) expands KKASSERT(lockstatus==LK_EXCLUSIVE). INVARIANTS kernels panic on every shutdown. Production no-op but SSET evaluation races with concurrent sysctl/notify. All other callers properly acquire lock. Root reboot/halt triggers. Recursive panic prevents crash dumps.