The evolution of programmable metasurfaces (PM) from passive beamforming to active information transmission marks a paradigm shift for next-generation wireless systems. However, this transition is hindered by fundamental limitations in conventional metasurface transmitter architectures, including restricted modulation orders, symbol-level spatial inconsistency, and significant harmonic interference. These issues stem from the intrinsic coupling between baseband signal processing and radio-frequency beamforming in monolithic designs reliant on simplistic switching mechanisms. This paper proposes a novel metasurface-enabled superheterodyne architecture (MSA) that fundamentally decouples these functionalities. The MSA introduces a dual-stage up-conversion process, comprising a digital up-conversion module for in-phase/quadrature modulation and baseband-to-intermediate frequency conversion, a precoder module for precoding, and a custom-designed magnitude-phase-decoupled metasurface that acts as a reconfigurable reflective mixer array. This decoupling of harmonic-free waveform generation from spatial precoding overcomes the critical drawbacks of existing approaches. Experimental results from a 5.8 GHz proof-of-concept prototype system validate the MSA's superior performance. The system generates spatially isotropic constellations for arbitrary-order QAM modulations, ensures consistent time-frequency signatures for applications like Doppler-spoofing, and achieves data rates up to 20 Mbps within a linear operating region that minimizes nonlinear distortion. The capability of employing spatial diversity and multi-stream interference cancellation has been demonstrated for the first time in a PM-based transmitter.