Integrated sensing and communication (ISAC) is evolving from dual-functional waveform and transceiver design toward an end-to-end network capability in which sensing observations are acquired, coordinated, processed, protected, and exposed to applications. This paper presents an architecture-centric synthesis of that evolution, organized using six operational contracts: input/output, uncertainty, timescale, decision, evaluation, and admissibility–applied across seven functional layers. These contracts provide a consistent basis for examining whether information produced at one layer remains valid, timely, interpretable, and implementable at the next. The survey covers physical-layer signal models; waveform and beamforming design; sensing metrics and communication–sensing tradeoffs; cooperative and service-oriented ISAC; intelligence and edge computation; propagation and hardware constraints; security and trust; and standards, datasets, prototypes, and testbeds. We pay particular attention to metric compatibility, uncertainty propagation, information age, computation and deadline reliability, sensing-service exposure, and the distinction between achieved performance and feasible operating points. The strength of reported results depends on their supporting evidence, ranging from analytical formulations and simulations to hardware measurements, real-time prototypes, and networked testbeds. A representative literature audit and research-gap matrix identify recurring requirements for cross-layer operation, including the transfer of estimator uncertainty and calibration validity, synchronization, characterization of latency tails and deadline violations, and feasibility checks for learned actions. The resulting synthesis identifies end-to-end interface design, trustworthy decision making, and reproducible validation as central requirements for deployable ISAC systems.