Function render_ac_accuracy 

pub fn render_ac_accuracy(results: &[AcResult], th: &AcThresholds) -> String

Render the AC-accuracy table for a set of per-circuit results.

Mirrors render_bias_accuracy, but for the AC SIGNAL instead of the DC operating point. One summary row per circuit — LEVEL (gain@f), the gain-normalized SHAPE residual (rms + phase-immune spectral), THD (WDF vs ngspice) and its Δ, the response tilt (max−min gain across the sweep), and the verdict — followed by the per-harmonic breakdown and the full frequency sweep. Returns the rendered string so tests can print AND assert on it.