Replace single-sample max/min proxy with mathematically proper sigma_1/sigma_2
ratio. Maintains a host-side VecDeque of up to 64 Q-value samples; once ≥ 8
samples are available computes the n_cols×n_cols Gram matrix X^T X, then
extracts the two largest singular values via power iteration + rank-one
deflation. Falls back to the coarse max/min ratio until the buffer fills.
compute_q_spectral_gap changed to &mut self; callers updated accordingly.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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