feat: add exp_cronbach_alpha reliability analysis (#14309)

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Summary

Adds exp_cronbach_alpha() — Cronbach's Alpha / reliability analysis — powering the new Cronbach's Alpha Analytics type in the desktop app (tam PR to follow).

Supports Pearson (Cronbach's Alpha), Polychoric (Ordinal Alpha), and Mixed Correlation (Mixed-Correlation Alpha), with an auto selector that picks the method from the item types. Reuses psych (already an Imports dep — no new dependency).

Contract

• exp_cronbach_alpha(df, ..., correlation_method = "auto", check_keys = FALSE, smooth = TRUE, correct = 0.5, max_nrow = NULL, seed = 1) — returns a model_df (class cronbach_alpha_exploratory), one model per group.
• glance(model) — overall alpha, standardized alpha, CI, G6, counts, average inter-item correlation, correlation type, interpretation.
• tidy(model, type = ...) — summary / alpha_if_dropped / item_stats / correlation / response_distribution / scale_candidates / data.

Testing

tests/testthat/test_reliability.R covers the Pearson, all-ordinal, mixed, reverse-key, and edge-case contracts:

• Pearson alpha matches psych::alpha's raw_alpha exactly; every section shape verified.
• Auto→Polychoric (ordered factors) → Ordinal Alpha; raw.r hidden.
• Mixed correlation → Mixed-Correlation Alpha; CI and raw.r are hidden by specification.
• <2 variables error; nominal-factor error; constant-column drop; group_by.

Notes

• NAMESPACE updated with the 3 new exports (exp_cronbach_alpha, tidy/glance S3 methods). devtools::document() mis-parses in this local env due to an unrelated find_data load error, so the entries were inserted by hand exactly as document() would emit them — please regenerate in CI if preferred.
• DESCRIPTION version bumped 16.0.5 → 16.0.6. The bundled binary must be rebuilt + the tam requiredRPackagesReduced.json bumped before the desktop feature works end-to-end.

🤖 Generated with Claude Code

Specification alignment follow-up

• Pearson exposes Feldt/Duhachek 95% CI and raw.r.
• Polychoric/Mixed omit the CI summary row and the raw.r output column.
• check.keys is applied consistently to matrix statistics, item statistics, and alpha-if-dropped results.
• Local full test execution still requires the repository's 4.6 devtools/exploratory dependency stack.