Projection-Defined Physicality in PT-Symmetric Quaternionic Spacetime

Generalized geometric frameworks can admit enlarged kinematical structure without thereby specifying a physical observable sector. In such settings, physical observability cannot be identified naively with the full kinematical space, because normalization, expectation values, conserved transport, and admissible evolution require a controlled criterion of physical sector selection. This paper formulates PT-symmetric quaternionic spacetime (PTQ) as a projection-defined physical framework built around that requirement. We state the minimal principles of PTQ, argue that physicality is defined only after projection onto an admissible observable sector, and show how the physical inner product, probability/current structure, and observable dynamics are to be understood at the framework level only after that projection has been imposed. Probability is treated as an induced structure of the projected sector rather than as a primitive assignment on unrestricted kinematics, while dynamics are formulated as constrained projected geometric evolution compatible with admissibility, metric consistency, and continuity. We also state a framework-level notion of falsifiability, centered on the requirement that a single projection-induced residual structure remain consistent across distinct observational regimes. The scope of the paper is deliberately limited: it does not present a full cosmological model, does not provide a full replacement for quantum theory, and does not claim a universal closed dynamical system. Its purpose is to supply a foundational statement of the PTQ program on which later technical, probabilistic, and empirical developments can be assessed.