feat(Data/Generalizations/Projectivity): hub + SolstadBott2024 overhaul

[hawkrobe]

Graduates the projection-prediction machinery from #963 into a shared Data/Generalizations/Projectivity hub, and overhauls the second projective-content paper (Solstad & Bott 2024, occasion verbs) onto it. Data verified against the paper PDF.

• Data/Generalizations/Projectivity: theory-neutral pool — ProjectionAccount signature, ProjectionDatum adapter (reads either atIssueness or notAtIssueness from paperFeatures), 27 artifact-sourced rows pooled from both papers' Data/Examples JSON, and predictionError/predictsWithin. Clears the ≥2-papers / ≥2-accounts bar.
• SolstadBott2024: drops the regression/Datum stats structs + native_decide; keeps the genuine structural theory (symmetric filtering and cataphoric resolution via FilteringDirection/ContextPolarity, Class C, Heim/Schlenker filtering, the IC bridge); adds the cross-paper result that occasion verbs project above the GPP diagonal — the mirror of TBD2018's establish. 558→265 lines.
• TonhauserBeaverDegen2018: retrofitted to consume the hub (local prediction layer removed; GPP theory + accounts kept).
• Bib: adds verified mandelkern-etal-2020; fixes two stale sources paths.

No native_decide/sorry; axiom-clean. CI runs the authoritative full build.