Simplify lt.R using lt predicate selectors and spanner auto-reorder (next lt release)

[yihui]

Context

PR #372 replaced gt with lt for summary tables. To work with the current CRAN lt (0.4), R/lt.R had to work around two lt limitations:

1. No pattern-based column selection — spanner/label columns are enumerated with grep("_time$", names(x), value = TRUE) etc.
2. Spanner columns had to be listed in visual (post-lt_move) order — lt matched a spanner to the visual position of its first column and spanned the next length(columns) cells, so a mismatched order silently dropped later spanners (this caused the "Probability of crossing efficacy bounds under H1" spanner to go missing; fixed in 76a63ca by listing columns in explicit asy-before-sim order).

Both limitations are addressed in the next lt version (merged upstream in yihui/lt#5 plus follow-up yihui/lt@50c09bc):

• Predicate formulas: columns = ~ endsWith(., "_time"), ~ grepl("_prob$", .), ~ . (all columns), etc.
• lt_label() accepts a single named list/vector of labels (no more do.call() splice).
• Spanner columns auto-reordered to body order at render, so their listed order no longer matters (predicate selectors are safe for spanners too).

Task (do this once lt >= that version is on CRAN)

In R/lt.R:

• Replace the grep(..., names(x), value = TRUE) spanner enumerations (and the explicit c("asy_time", "sim_time") ordering added in 76a63ca) with predicate selectors, e.g. lt_spanner(Time ~ endsWith(., "_time")). Spanner auto-reorder means order no longer needs hand-managing.
• Replace the do.call(lt::lt_label, c(list(ans), labels)) splice with ans |> lt::lt_label(labels) (single named vector). See the review suggestion on PR Replace gt with lt for summary tables (#371) #372.

In vignettes/discrepancy-between-simtrial-and-survival.Rmd:

• Replace lt_format(columns = names(fr)[sapply(fr, is.numeric)], ...) with the everything() equivalent columns = ~ ..

In DESCRIPTION:

• Bump the lt Imports version constraint to the release that ships these features (lt (>= X.Y.Z)).

References

• gt→lt PR: Replace gt with lt for summary tables (#371) #372
• lt predicate + lt_label PR: Support predicate formulas in column selection yihui/lt#5
• lt spanner auto-reorder: yihui/lt@50c09bc
• Missing-spanner fix that this would supersede: commit 76a63ca