Partition at least four vertices into at least two nonempty parts. Assign Coxeter exponent 2 within each part, a fixed exponent m >= 5 between parts, and replace selected cross-part exponents by 3. Suppose each vertex has at most one replacement neighbor in any other part. If the replacement graph has maximum degree Delta and (cos(pi/m) - 1/2) Delta < 1, the normalized Coxeter form has exactly one negative eigenvalue and no zero eigenvalue, with an explicit lower bound for its positive eigenvalues. The uniform degree caps are 3 for m = 5 and 2 for every finite m >= 6; both are sharp. A paired-part construction gives the entire obstruction spectrum. The diagrams avoid A3 and B3, so the established rewriting theorem of Blasco-Garcia, Cumplido, Holt, Morris-Wright and Rees supplies a quadratic word algorithm for each fixed Artin presentation. Nonstar examples have unboundedly many exponent-3 pairs. This is a complete spectral construction and an application of an existing algorithm, not a new general Artin word algorithm or a resolution of the small-type problem. The manuscript is AI-assisted, self-audited and unrefereed; no absolute priority claim is made.
