The Broken Symmetry is an Integer Fraction: A 3:5:5 Trace Partition, Hypercharge Lexicon, and the 3/13 Electroweak Mixing Skeleton in TWIST-J

Standard electroweak theory leaves the weak mixing angle sin²θ_W as a continuous parameter fixed by experiment. This paper proves a finite arithmetic theorem in the coefficient shadow 𝔽₅⁴ of the cyclotomic ring ℤ[ζ₅] from TWIST-J and uses it to extract a structural skeleton for hypercharge and the weak mixing ratio. The Klein four-group V₄ = ⟨b, d⟩, generated by piston swap‑and‑negate and by the affine reflection d(ψ) = 2p₀ − ψ with p₀ = (1, 3, 4, 2), has 169 = 13² orbits. These orbits split under the coefficient trace into 39 : 65 : 65 = 3 : 5 : 5. Under the decoder dictionary Y_F = (B − L) + 2T₃ᴿ, with quark baryon number B_quark = 1 / dim ker(Tr) = 1/3, the finite carrier ker(Tr) ⊂ 𝔽₅⁴ gives the color dimension and the dictionary reproduces the full first‑generation Standard Model hypercharge table, including the optional sterile ν_R and the Higgs doublet. Assigning the trace‑zero sector S₀ to the U(1)_Y channel and the paired nonzero‑trace sectors S₊ ∪ S₋ to the SU(2)_L channel, and assuming the explicit orbit‑channel coupling rule gᵢ² ∝ Nᵢ, the partition yields g′² / g_w² = 3/10 and therefore the tree‑level value sin²θ_W(tree) = 3 / (3 + 10) = 3/13. Adding the canonical TWIST‑J Ω‑projection X = 1/(32π²φ⁴) built from the spectral data of the multiplication matrix M_J gives sin²θ_W(M_Z) = 3/13 + 1/(32π²φ⁴) = 0.231231186… This lies +1.12 × 10⁻⁵ above the PDG 2025 MS value ŝ²_Z(M_Z) = 0.23122 ± 0.00006, corresponding to +0.19 σ or +48 ppm relative. The comparison is made to the MS Z‑pole parameter, not to the effective leptonic weak mixing angle. The paper does not claim a first‑principles derivation of the electroweak action, Higgs‑sector dynamics, electroweak mass generation, or renormalization‑group running. The action‑density origin of the coupling rule gᵢ² ∝ Nᵢ and the scheme map from the TWIST‑J Ω‑projection to the MS renormalization prescription remain explicit open obligations. The result is a finite arithmetic electroweak skeleton: the integer structure is closed; the action‑level completion is the next test.