fix: resolve locked namespace binding in civilytics_load_fonts() and default theme to transparent background

- Replace .cv_fonts_loaded <<- TRUE with environment-based state
  (.cv_state) to avoid 'locked namespace binding' error (#18)
- Add force=FALSE argument for idempotent font loading with guard check
- Flip theme_civilytics() paper_bg default to FALSE (transparent) (#7)
- Update roxygen docs and examples for both changes

R/fonts.R

@@ -5,32 +5,45 @@ CV_FONT_SANS    <- "Inter"           # axis text, legends, UI elements
 CV_FONT_SERIF   <- "Source Serif 4"  # body prose / editorial long-form
 CV_FONT_MONO    <- "JetBrains Mono"  # code, data tables, numeric callouts
 
-# Internal flag so civilytics_load_fonts() is idempotent within a session.
-.cv_fonts_loaded <- FALSE
+# Mutable package state held in an environment so the binding itself stays
+# locked (R locks all namespace bindings at load time) while the contents
+# remain writable. See https://adv-r.hadley.nz/environments.html#environments-as-containers
+.cv_state <- new.env(parent = emptyenv())
+.cv_state$fonts_loaded <- FALSE
 
 #' Load Civilytics brand fonts
 #'
-#' Downloads Inter and Libre Franklin from Google Fonts via
-#' [sysfonts::font_add_google()], then calls [showtext::showtext_auto()] so
-#' that all graphics devices render text with those fonts. This is called
-#' automatically when the package loads; use this function to retry if the
-#' initial load failed (e.g., the machine was offline at load time).
+#' Downloads Inter, Libre Franklin, Source Serif 4 and JetBrains Mono from
+#' Google Fonts via [sysfonts::font_add_google()], then calls
+#' [showtext::showtext_auto()] so that all graphics devices render text with
+#' those fonts. This is called automatically when the package loads; use this
+#' function to retry if the initial load failed (e.g., the machine was offline
+#' at load time).
 #'
-#' @return Invisibly returns `NULL`.
+#' Subsequent calls within the same session are no-ops unless `force = TRUE`.
+#'
+#' @param force Logical. If `TRUE`, reload fonts even if they were already
+#'   loaded in this session. Default `FALSE`.
+#'
+#' @return Invisibly returns `TRUE` if fonts were loaded, `FALSE` if skipped
+#'   (already loaded and `force = FALSE`).
 #' @export
 #'
 #' @examples
 #' \dontrun{
 #' civilytics_load_fonts()
 #' }
-civilytics_load_fonts <- function() {
+civilytics_load_fonts <- function(force = FALSE) {
+  if (.cv_state$fonts_loaded && !force) return(invisible(FALSE))
+
   sysfonts::font_add_google("Inter",          family = "Inter")
   sysfonts::font_add_google("Libre Franklin", family = "Libre Franklin")
   sysfonts::font_add_google("Source Serif 4", family = "Source Serif 4")
   sysfonts::font_add_google("JetBrains Mono", family = "JetBrains Mono")
   showtext::showtext_auto()
-  .cv_fonts_loaded <<- TRUE
-  invisible(NULL)
+
+  .cv_state$fonts_loaded <- TRUE
+  invisible(TRUE)
 }
 
 .onLoad <- function(libname, pkgname) {