Structural Gravitational Waves v1.9: Precision Ringdown — High-Resolution QNM Spectroscopy

Structural Gravitational Waves (SGW) v1.9 presents an illustrative precision-ringdown framework that extends the SGW series from the catalog-scale architecture established in v1.8 to high-resolution quasi-normal-mode (QNM) spectroscopy of individual gravitational-wave events. This release develops a dedicated methodological framework for precision ringdown analysis, including an extended structural QNM waveform model with overtones, mode mixing, and second-order (nonlinear) QNMs; a five-layer hierarchical inference architecture; population spectroscopy; no-hair consistency mapping; residual spectroscopy; joint gravitational-wave and electromagnetic constraints; a parameter-wise systematics budget; an eight-test validation suite (R1–R8); detector precision forecasts; and an illustrative scientific-reach assessment. The accompanying paper contains sixteen figures covering the complete precision-ringdown workflow, from waveform modeling through inference, validation, detector forecasts, and the roadmap toward the SGW v2.0 Structural Gravity Precision Framework. All numerical values, forecasts, validation outcomes, and scientific projections presented in this release are explicitly illustrative and are intended to demonstrate the framework architecture rather than represent results from actual LIGO–Virgo–KAGRA or future gravitational-wave observations. This release continues the Structural Gravitational Waves research program by providing the dedicated precision-spectroscopy layer that bridges the global inference architecture of SGW v1.8 and the planned SGW v2.0 Structural Gravity Precision Framework.