TQNT-V5.16 — Reheating from the χ-field and internal BBN tension diagnostic

TQNT-V5.16 derives the reheating mechanism after R²-Starobinsky-like inflation (V5.14/V5.15) from the χ-field decay rate Γreh = αEM18 · Hend, where the exponent 18 is the Vassiliev complexity of the dominant nucleon channel (cn = 18). The number of e-folds is predicted to be Ne = 35.8 (w = 0, matter-like χ) or Ne = 50.5 (w = 1/3, radiation-like χ). Honest diagnostic — internal tension reported: Reheating temperature Treh = 0.15 MeV, BELOW the BBN lower bound (≈ 1 MeV) by a factor of ~7. Recomputed scalar spectral index ns is incompatible with Planck at -5σ (w = 0) or -1.1σ (w = 1/3). This is a genuine internal diagnostic of TQNT, not a tuning step. A proposed resolution via a two-stage cascade is deferred to V5.17. This paper is released as a transparent record of the BBN tension and of the parameter-free reheating prediction it produces. Complements the V5 monograph (10.5281/zenodo.19131538) and follows V5.14 (R² correction) and V5.15 (R² coefficient derivation).