Add vignette: modular TLG pipeline (compute → tabulate → customize)

[Melkiades]

Motivation

The get_cox_pairwise_df() → tbl_coxph() → modify_*() pattern demonstrates crane's modular design: statistics are computed separately from tabulation, and formatting can be customized at each layer. This pattern applies broadly across crane but is not documented in a user-facing way.

Proposed vignette

A short vignette (vignettes/modular-pipeline.Rmd) covering:

1. The three layers

• Compute — functions like get_cox_pairwise_df() that produce a statistics data frame
• Tabulate — functions like tbl_coxph() that render the data frame as a gtsummary table
• Customize — modify_fmt_fun(), modify_header(), pre-formatting columns, .format_pvalue() pass-through

2. When to set digits / formatting at each layer

• At compute time: HR and CI precision is set by get_cox_pairwise_df() via sprintf("%.2f", ...)
• At tabulate time: p-value precision is set by .format_pvalue() inside tbl_coxph()
• After the fact: users can pre-format the p-value column as character (pass-through) or use modify_fmt_fun() on the gtsummary output

3. Other modular components

• tbl_hierarchical_rate_by_grade() + add_grade_column() — decoupled styling
• tbl_roche_subgroups() + add_forest() — decoupled forest plot
• tbl_with_pools() — pooling wrapper that works with any .tbl_fun

4. Swapping layers

• Bring your own data frame to tbl_coxph() (e.g., from a different statistical engine)
• Use a different .tbl_fun inside tbl_roche_subgroups()

Ref: discussion in #245