WHY A CONFIG-DRIVEN DEFAULTS LIST RATHER THAN LITERALS SCATTERED THROUGH
EACH FIGURE FUNCTION: re-theming used to mean editing R code at a dozen
call sites, with no guarantee of catching every one. Every figure function
below takes a colors = fcs_colors() parameter (default arguments are resolved
at CALL time in R, so a run that overrides COLORS via –config, see
apply_color_config() in main(), changes what every later figure call
picks up with no other code change) and reads exclusively from this list –
no hex code or named colour is hardcoded anywhere past this point. Override
any subset from –config's colors: block; unset keys keep these defaults
(see write_config(), which round-trips the resolved list back out as YAML).