docs: add panel data example to lag_run.Rd for Stata migrants

[LeonidasZhak]

Summary

Adds a panel data example to lag_run.Rd showing the correct pattern for lagging within groups — the most common use case for Stata users migrating to R.

Stata equivalent:

xtset firm year
gen lag_sales = L.sales

runner equivalent (new example):

df <- data.frame(
  firm = rep(c(A, B), each = 3),
  year = rep(2010:2012, 2),
  sales = c(100, 110, 125, 200, 215, 230)
)
library(dplyr)
df <- df %>%
  arrange(firm, year) %>%
  group_by(firm) %>%
  mutate(lag_sales = lag_run(sales, lag = 1)) %>%
  ungroup()

Also adds irregular time index examples demonstrating the idx and nearest parameters.

Rationale

The existing lag_run.Rd had only 3 basic vector examples with no guidance on panel data usage. Panel data lagging is the single most common migration scenario from Stata, where L.var after xtset automatically handles group boundaries and time ordering.

Validation

• tools::checkRd('man/lag_run.Rd') passes
• All example code runs successfully
• All 1043 tinytest tests pass
• 3 files changed, 57 lines added

[LeonidasZhak]

Withdrawing this small automated PR while I consolidate an oversized batch of contributions and reduce maintainer review burden. Sorry for the noise, and thank you for maintaining the project.