Modelling stage: fit PCA/PLS from a workset, with the Hangar and Logbook

[kgdunn]

Goal

Turn the draft preprocessing spec into fitted model variants and surface the results.

Scope

• Fit PCA/PLS from a dataset + PreprocessingSpec (the apply_spec path already exists for PCA); add PLS.
• Model diagnostics UI (ECharts): scores with Hotelling's T² ellipse, loadings, SPE/DModX, contributions, VIP, R²X/R²Y, cross-validated Q².
• Hangar: the collection of all model variants a user has built.
• Logbook: each model's provenance — preprocessing, excluded samples, validation history — using the parent_id lineage.
• Create a new variant from an existing one by tweaking its spec (different transforms/exclusions), demonstrating lineage.

Depends on #6 for persistence. Roadmap item — keep open.