feat(psychometric): restore Driver p.16 discreteDIFFUSIONstd Q_Δt/p on main

[seonghobae]

Restores Driver, Oud, and Voelkle (2017, p. 16) scalar discreteDIFFUSIONstd Q_Δt / p onto current main after 0ce16e8 dropped the pre-consolidation code while research notes already named the map (register items 57–58).

Ports the #296 slice onto 6444a81. Does not cherry-pick the conflicting #296 commits.

Form strictly positive asymDIFFUSION p = −q / (2 a) first, then Q_Δt from Equation 4, then the ratio. Scalar stationary identity is 1 − exp(2 a Δt). The 2017-era summary.ctsemFit.R forms unstandardised discreteDIFFUSION when verbose=TRUE and does not form a discreteDIFFUSIONstd matrix. Printed p. 16 example is discreteDRIFTstd. Footnote 4 uses only the relevant within-subject variance.

Fail-closed: zero q, a ≥ 0, non-event clock, non-positive Δt.
Refuse: unstandardised Q_Δt, continuous q/p = −2 a (DIFFUSIONstd), trait-contaminated Q_Δt / (trait + p + added).

Supersedes conflicting #296. Does not invent DIFFUSIONstd (register 59). Does not convert #310.

Local cargo test -p psychometric_core is green. Meredith (1993) and Mislevy (1991) remain unread (Unpaywall is_oa: false). Still not a Kalman filter, not a matrix expm, not ESEM estimation, not DSEM, and not ctsem estimation.

Head is c73b250. Do not merge without independent APPROVE and green product CI. Stays Draft.

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[seonghobae]

Requesting Copilot review of discreteDIFFUSIONstd Q_Δt/p restore.

[seonghobae]

@github-copilot review this exact head c73b250. Requesting Copilot code review of the discreteDIFFUSIONstd restore.