Further split _sec-predictive-margins.qmd (~325 lines) into g-computation / margins-defs / subgroup subfiles

[d-morrison]

_subfiles/logistic-regression/_sec-predictive-margins.qmd is ~325 lines, above the ~100-line CLAUDE.md guideline. It was created in #706 when _sec-marginal-vs-conditional.qmd (636 lines) was split; this is a second-level follow-up split.

Three natural blocks (each ~80–110 lines):

1. G-computation — #def-g-computation → #thm-g-computation-consistency → WCGS causal example → e.g. _sec-g-computation.qmd.
2. Predictive-margin definitions — #def-predictive-margins, #def-full-sample-predictive-margin, #cor-full-sample-pm, #exm-full-sample-pm → e.g. _sec-predictive-margins-defs.qmd.
3. Subgroup margin — #def-subgroup-predictive-margin, #thm-subgroup-pm, #exm-subgroup-pm, #lem-subgroup-pm-proportion, WCGS observed example → e.g. _sec-subgroup-predictive-margin.qmd.

Separate from #938 (which tracked _sec-marginal-vs-conditional.qmd / _sec-causal-marginal-risk.qmd). Pure mechanical move; cross-references resolve within the rendered chapter regardless of file boundaries.

[github-actions]

The issue proposes splitting the _subfiles/logistic-regression/_sec-predictive-margins.qmd file (currently ~325 lines) into three smaller subfiles to better align with the ~100-line guideline outlined in CLAUDE.md. This file was previously created during a first-level split in #706. The suggested breakdown includes: (1) a g-computation section for definitions, a theorem, and examples; (2) a section for predictive-margin definitions and associated examples; and (3) a subgroup margin section containing definitions, theorems, examples, and an observed case study. This is described as a purely mechanical change where cross-references will remain functional, distinguishing it from related issue #938.