Paper Lc — Cadence-Coherence-Need as Structural Source of Relativity: Lorentz IS Cadence-Relative Motion. The Lorentz factor γ = 1/sqrt(1 − v²/c²) admits a structural reading within the Light Frame Cadence Theory (LFCT) relational ontology: it IS the per-locus mutual cadence-coherence-need at quadratic order in the A2 representability cost. Under the LFCT Foundation Axioms (concept DOI 10.5281/zenodo.20306153), cadence-coherence is the default state at every locus, and any departure carries a representability cost quadratic at leading order by A2. When two loci establish mutual cross-readability, the mutual coherence-need scales quadratically with the relative-motion deviation from the cadence-aligned light reference. The Lorentz factor IS the structural cost of that mutual coherence-need at quadratic order — not an imposed kinematic rescaling but the structural-coherence cost expressed at relativistic invariant form. Special relativity's empirical phenomenology is preserved exactly. The paper does not contest Einstein 1905's derivation, does not modify the Lorentz transformations, and does not propose new measurable kinematic effects. It identifies the structural source the mainstream calculation describes — the recurring LFCT pattern that LFCT names what mainstream fragments describe while mainstream continues to supply the observable bookkeeping. The sheet-music view supplies the intuitive frame: each locus carries its own cadence sequence; cross-locus translation requires the sheet-music conversion that aligns one cadence to the other; the Lorentz factor IS that conversion ratio. Under cadence-coherence-need the symmetric Lorentz transformation reads as two loci paying the symmetric cost of mutual readability — cause and effect become mutual. The Lorentz factor sits within a family-articulation architecture in which multiple framework carriers reach the same structural address through different observable face presentations: the Schwarzschild factor at the gravitational face, the nuclear-coherence potential V(chi) at the nuclear face, and the fine-structure-constant form 1/alpha at the atomic face. The family is organised by the Core MF 1-2-3-(+4) Metapattern Theorem. Companion documents: LFCT Foundation Axioms (concept DOI 10.5281/zenodo.20306153), LFIS-34 Relational Ontology (concept DOI 10.5281/zenodo.20268822), and the LFCT Core trilogy — Conceptual Spine + Mathematical Framework (concept DOI 10.5281/zenodo.18903197).