The Folding Real Form Is Split E8(8) (T265): The T172 Eight-Quantum-Number Particle Spectrum Requires Real Rank Eight, Strongly Favouring the Split Form Over the Quaternionic and Fixing the Freeze at t^(-1/7), Modulo the Casimir-Invariant versus Cartan-Weight Reading of the Quantum Numbers

Photonic Universe Hypothesis (PUH) — T265 — STRUCTURAL RESOLUTION NOTE. T264 excluded the compact real form E8(-248) as the folding form (a definite Killing form has no descent directions, which a directed cascade requires), removing the frozen p=124, t^(-1/123) branch and leaving a residual between the two indefinite real forms: split E8(8) vs quaternionic E8(-24). This note resolves the residual in favour of split, on a ground stronger than T264's two opposed considerations, and names the one technical point it rests on. THE GROUND: T172 identifies the eight E8 Casimirs with eight physical quantum numbers (mass, spin, colour, weak isospin, hypercharge, lepton number, baryon number, topological charge) and defines a particle as a state with all eight fixed. For eight quantum numbers to be simultaneously specifiable as REAL observables, the algebra needs eight independent simultaneously-real commuting directions — a real Cartan subalgebra of dimension eight, i.e. REAL RANK EIGHT. REAL RANKS of the E8 real forms: - Compact E8(-248): real rank 0 (already excluded, T264). - Quaternionic E8(-24): real rank 4 — only four simultaneously-real quantum numbers; the other four complex-paired. - Split E8(8): real rank 8 — all eight simultaneously real, all 240 roots real. RESOLUTION: only the split form has real rank eight. The quaternionic form (real rank four) cannot carry T172's eight-quantum-number spectrum and is disfavoured; with compact already excluded, the SPLIT form E8(8) is the real form consistent with the framework's commitments. The freeze is then the split-form law p=8, u(t) ~ t^(-1/7) (live, non-frozen) — fixed WITHOUT the substrate equation of state. The choice T251 framed as the equation-of-state question is settled by the particle spectrum (T172) plus the cascade's directedness (T264). CHIRALITY DISSOLVES: T264's pull toward quaternionic came from T212's parity violation. But T212 derives parity violation from the REBOUND breaking SU(2)_R at a high scale — a SPONTANEOUS breaking WITHIN the form (T136/T155), not an intrinsically chiral ambient form. The split form fully accommodates spontaneous rebound-induced parity violation, so chirality supplies no pull toward quaternionic once the breaking is located as dynamical. THE ONE TECHNICAL POINT (flagged honestly): the inference reads all eight of T172's quantum numbers as simultaneously-real CARTAN observables (real rank eight). In standard Lie theory, physical quantum numbers are a MIX of Casimir INVARIANTS (constant on a representation — e.g. mass) and Cartan WEIGHTS (varying within it — e.g. colour/hypercharge components). The real-rank-eight requirement is strictly forced only if all eight are simultaneously-diagonalisable Cartan-type quantities, not a mix. If some are Casimir invariants (real on any form), the requirement weakens and the quaternionic form could reopen. Determining each quantum number's Cartan-vs-Casimir character is the calculation that would make this a theorem. Until then the split form is the STRONGLY FAVOURED resolution, conditional on the simultaneously-real reading. Completes the thread T251 -> T262 -> T264 -> T265. Found via full-corpus search (T172 was the decisive structure, two papers from T264).