Nested case when

[EmilHvitfeldt]

In reference to tidymodels/orbital#109

This PR switches all tree-based models to generate nested case_when() expressions instead of flat ones. Nested expressions mirror the tree structure, producing more efficient R and SQL code.

Before (flat):

case_when(
  x <= 5 & y <= 3 ~ "low",
  x <= 5 & y > 3  ~ "med",
  x > 5           ~ "high"
)

After (nested):

case_when(
  x <= 5 ~ case_when(
    y <= 3 ~ "low",
    .default = "med"
  ),
  .default = "high"
)

Changes

New features

• All tree models (rpart, partykit, ranger, randomForest, xgboost, lightgbm, catboost) now generate nested case_when() expressions
• New vignette (vignettes/tree-internals.Rmd) explaining the nested approach and tree algorithms
• parse_model() documentation now explains the version system (v1/v2/v3) and model type classes

Internal improvements

• Reorganized model files with clear section headers separating nested builders from legacy code
• Extracted shared helpers to reduce duplication:
  • apply_xgb_objective() and apply_lgb_objective() for objective transformations
  • apply_lgb_multiclass_transformation() and apply_catboost_multiclass_transformation() for multiclass handling
• Documented operator naming conventions in tree-nested.R
• Legacy flat case_when() builders preserved for backwards compatibility with parsed models saved using v1/v2 format