docs: add grouped rolling mean example to runner.Rd

[LeonidasZhak]

Summary

Added a panel data example to the runner.grouped_df method in runner.Rd showing how to compute rolling means within groups using dplyr.

Stata equivalent:

bysort firm (year): rolling mean_sales = r(mean), window(3) clear: summarize sales

runner equivalent (new example):

library(dplyr)
df <- data.frame(
  firm = rep(c(A, B), each = 5),
  year = rep(2010:2014, 2),
  sales = c(100, 110, 125, 130, 145, 200, 215, 230, 250, 270)
)
df %>%
  arrange(firm, year) %>%
  group_by(firm) %>%
  mutate(roll_mean = runner(sales, f = mean, k = 3)) %>%
  ungroup()

Rationale

The runner.grouped_df method had no examples. Rolling operations within groups (panel data) is the single most common migration scenario from Stata, where bysort + rolling automatically handles group boundaries. The runner() function with dplyr's group_by provides the equivalent functionality.

Validation

• tools::checkRd('man/runner.Rd') passes (no output = no errors)
• All 1043 tinytest tests pass
• 2 files changed, 31 lines added

[gogonzo]

Thanks!