Replace bare backticks with cli inline markup across all messaging calls

- {.arg} for argument names in stop_/warning_/message_ calls
- {.cls} after "of class" text in format_class() and elsewhere
- {.fun} for function names (replaces `fn()` pattern)
- {.pkg} for tidyverse package names (dplyr, ggplot2)
- {.code} for code literals (TRUE, FALSE, expressions)
- Rewrite print.ab: use cli named-vector with * bullets and code
  highlighting when cli >= 3.0.0; keep plain-text fallback otherwise
- Fix typo in as.sir(): "of must be" -> "or must be"
- switch sir.R verbose notes from message() to message_()

https://claude.ai/code/session_01XHWLohiSTdZvCutwD7ag2b

R/ggplot_sir.R

@@ -295,7 +295,7 @@ geom_sir <- function(position = NULL,
                      ...) {
   x <- x[1]
   stop_ifnot_installed("ggplot2")
-  stop_if(is.data.frame(position), "`position` is invalid. Did you accidentally use '%>%' instead of '+'?")
+  stop_if(is.data.frame(position), "{.arg position} is invalid. Did you accidentally use {.code %>%} instead of {.code +}?")
   meet_criteria(position, allow_class = "character", has_length = 1, is_in = c("fill", "stack", "dodge"), allow_NULL = TRUE)
   meet_criteria(x, allow_class = "character", has_length = 1)
   meet_criteria(fill, allow_class = "character", has_length = 1)