fix(psychometric): cover MANIFESTVARstd Display claim-boundary arms

[seonghobae]

Summary

Recovery tests already constructed the four Driver p. 16 MANIFESTVARstd claim-boundary errors (θ = 0, unstandardised θ, MANIFESTTRAITVARstd, Equation 5 Var(y)) but never executed Display. Nightly unique-line coverage on #351 listed error.rs 1227/1230/1233/1236. This PR only runs those match arms.

Not a new map. Not stacked on #351 / #350 / #310. Meredith (1993) and Mislevy (1991) remain unread (Unpaywall 2026-08-31T06:36Z: is_oa: false). Still not a Kalman filter, not a matrix expm, not ESEM estimation, not DSEM, and not ctsem estimation.

Head is b460d42101e30a7ca55a54f17951ffdfc4349677 on main 1bc02f580cf48e1d39da239f0e818453437c31c3.

Verification

• cargo fmt -p psychometric_core -- crates/psychometric_core/src/error.rs
• cargo test -p psychometric_core --lib standardised_manifest_variance_boundary_messages_are_stable
• cargo clippy -p psychometric_core --all-targets -- -D warnings

Do not self-approve. Do not merge without independent non-author APPROVE + exact-head required-check success.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[cwl-noema-review]

Noema LLM review

The PR introduces several changes to the psychometric core, including the recovery of standardised manifest-trait variance, and the test cases for standardised manifest variance boundary messages have been updated to be more comprehensive and robust.

Findings

• [low] crates/psychometric_core/src/error.rs:2073: The code could benefit from additional comments to explain the purpose of the test cases and the expected behavior of the code.

• Result: APPROVE

• Head SHA: b460d42101e30a7ca55a54f17951ffdfc4349677

• Reviewer credential: noema-review-github-app

• Actor: cwl-noema-review[bot]

[seonghobae]

Restack on protected main (a243f18)

Non-force merge of origin/main (merge commit 05f882d6); the only conflict was the CHANGELOG.md append, both entries kept.

Local evidence on the pushed head (toolchain 1.98.0): cargo test -p psychometric_core 363 passed / 0 failed, clippy -D warnings clean, cargo fmt --all --check clean, documentation/workspace/docstring contracts PASS, git diff --check clean.

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