The Dividing Line. Note on Fibonacci Entry Structure.

The construct–control–classify discipline is applied to a third territory: the Fibonacci entry points of primes. The rank of apparition z(p) — the least index k with p | F_k — is governed by the classical corridor law z(p) | p − (5/p), which splits the primes into two corridors according to the splitting behavior of x² − x − 1 modulo p. All 664,578 primes below 10⁷ are audited: corridor adherence is 100.0000% at every scale tested, cross-validated by brute-force iteration on an independent sample; a null battery drawing the corridor label from the wrong quadratic discriminants (3, 7, 11, 13) collapses to coin-flip adherence (49.97–50.01%); and the law fails for 99.82% of composite moduli, with the survivors reproducing the classical Fibonacci pseudoprimes exactly. The period multiplier is shown to satisfy M² ≡ (−1)^z (one line from Cassini's identity), and the valve resolves completely: π(p)/z(p) ∈ {1, 2, 4} is determined exactly by z(p) mod 4 — proved in two lines from the unit ω = φ/ψ — while the inert corridor never populates the z ≡ 2 (mod 4) class at all, an exact exclusion proved by 2-adic counting against the Frobenius action. The measured corridor-conditioned densities of maximal-rank primes lock onto the rational-Artin benchmarks 18A/19 and 20A/19 to within 6 × 10⁻⁴, bracketing Artin's constant symmetrically. This third territory is the first in the series with a measurable boundary: the corridor level is exact and provable, the distribution level is pinned to constants whose proof is GRH-conditional. The note maps that dividing line.