Strong sector, confinement, and hadron masses from boundary wave geometry

Paper 5 of the boundary wave framework series. Makes the strong sector explicit and extends the framework to composite hadrons. The fundamental string tension sigma_fund = g^2 C_F/2 = 1 fixes alpha_s = 3/(8pi) = 0.1194 (1.2% from measurement). Confinement is exact in 2D SU(N) Yang-Mills; the glueball spectrum from Casimir scaling matches lattice ratios at 8% RMS; the pure-gauge deconfinement temperature T_c = 277 MeV agrees with lattice data at 2.4%. A baryon is three S^1/Z_2 orbifold intervals at 120 degrees (Mercedes configuration); a meson is two orbifolds at 180 degrees. The meson binding scale K = N_c^2 T_F (m_s - m_d) is derived from the U(N_c) color channel count on the boundary, and the regime factors follow from the 2D YM Casimir structure. Predictions: 18 baryon masses (0.46% mean error, 16 sub-percent), 12 meson masses (0.20% mean error, 12/12 sub-percent), gauge boson widths, and neutron beta decay energy. The mass gap proof strategy is developed separately in Paper 6. ---Boundary Wave Framework Series (all submitted to Physical Review D):1. One formula for all fermion masses — https://zenodo.org/records/195787572. Decoherence spectrum of the Standard Model — https://zenodo.org/records/195787963. Fermion generations as standing wave nodes — https://zenodo.org/records/195788674. Electroweak masses and CP violation — https://zenodo.org/records/195789295. Strong sector, confinement, and hadron masses — https://zenodo.org/records/19579136