feat: expand color system to 53 colors/10 palettes, add grid/slide themes, and README gallery

Replace the 16-color / 3-palette system with the full Civilytics design
system: 53 named colors (full navy, ember, violet ramps plus teal, plum,
moss, brass supporting hues) and 10 visualization palettes (qual,
qual_warm, qual_cool, seq_ember, seq_navy, seq_violet, seq_paper_ink,
div_navy_ember, div_violet_ember).

Enrich theme_civilytics() with grid and paper_bg parameters, add
theme_civilytics_slide() for presentations, and load all four brand fonts
(Inter, Libre Franklin, Source Serif 4, JetBrains Mono).

Add README.Rmd with rendered gallery showing all palettes, theme
variants, grid options, dark/slide modes, facets, and scale usage.

NAMESPACE

@@ -7,6 +7,8 @@ export(civilytics_colors)
 export(civilytics_load_fonts)
 export(civilytics_logo)
 export(civilytics_pal)
+export(civilytics_palette)
+export(civilytics_palettes)
 export(clopper_pearson)
 export(countCleanr)
 export(countDots)
@@ -42,6 +44,7 @@ export(simpleCap)
 export(star_subs)
 export(theme_civilytics)
 export(theme_civilytics_dark)
+export(theme_civilytics_slide)
 export(trim_max)
 export(waldInterval)
 export(z_gap_test)