feat: bump title sizes, add font_scale to logo functions, ship SVGs

- Change rel_large default from 16/14 to 20/14 (~1.43x) to match the
  Civilytics editorial design system. Subtitle now renders at 1.0x base
  instead of 0.86x.
- Switch axis.text to rel() sizing so all text elements cascade from the
  root font_size, enabling uniform scaling.
- Add font_scale parameter (default 1.1) to civilytics_logo(),
  add_logo(), and add_logo_ga() — inflates text by ~10% before
  arrangeGrob composition to compensate for viewport shrinkage.
- Ship SVG logo variants (mark, wordmark, pulse) in inst/img/ for
  Quarto/HTML templates.
- Add "Maintaining brand assets" section to README documenting inst/img/
  file inventory and update workflow.
- Document full font size hierarchy in theme_civilytics() roxygen.

README.Rmd

@@ -256,3 +256,30 @@ The package also includes helpers for public-sector data analysis:
 | `clopper_pearson()` / `agresti_coull_interval()` | Proportion confidence intervals |
 | `match_test()` / `trunc_match()` | Fuzzy join diagnostics |
 | `perturb_count()` / `random_round()` | Privacy-preserving data perturbation |
+
+## Maintaining brand assets
+
+Logo and brand mark files live in `inst/img/`. The package ships both
+PNG (for ggplot2 raster composition) and SVG (for Quarto/HTML output)
+variants:
+
+| File | Format | Used by |
+|:-----|:-------|:--------|
+| `civilytics-wordmark.png` / `.svg` | Full "Civilytics" lockup | `make_logo_grob("wordmark", "light")`, Quarto templates |
+| `civilytics-wordmark-reverse.png` / `.svg` | Light-on-dark wordmark | `make_logo_grob("wordmark", "dark")`, dark slides |
+| `civilytics-mark.png` / `.svg` | Compact C-pulse icon | `make_logo_grob("mark", "light")` |
+| `civilytics-mark-reverse.svg` | Light-on-dark mark | `make_logo_grob("mark", "dark")` |
+| `civilytics-pulse.svg` | Standalone waveform glyph | Quarto slide footer chrome |
+
+To update the logos, replace the files in `inst/img/` with new versions
+using the same filenames. The PNG files must be raster images (the
+ggplot2 logo functions read them via `png::readPNG()`). SVG files are
+passed through as-is by Quarto and HTML templates.
+
+After replacing files, re-render the README gallery to update the
+screenshots:
+
+```r
+devtools::load_all()
+rmarkdown::render("README.Rmd")
+```