v2.15.2 — GNU/Linux build fix

Maintenance release.

Fix

• GNU/Linux build now compiles. The vendored msfa FM engine header (aligned_buf.h, used by Operator / OP ENV) referenced size_t without including <cstddef>; macOS and Windows toolchains provided it transitively, but strict Linux libstdc++ did not. Added the explicit include.

No functional or audio changes.