Redesign chart visualizations to match Civilytics white-paper style

Rebuilds all 6 charts around a validated categorical race palette, row-based
sex encoding, and a consistent observed-vs-modeled mark convention (diamond
vs. filled bar/density) instead of ad hoc per-chart color schemes. Adds a
shared student-group filter (defaults to all 8 groups) that scopes every
chart's data from one place in ChartPanel.

Drops the D3 dependency entirely in favor of plain SVG, removing the
imperative-DOM bug class behind this app's repeated "fix the fix" commits.
Replaces the fake symmetric-normal posterior approximation with a skewed,
median-preserving fit to the API's interval bounds, clearly labeled as an
approximation. Fixes two broken SVG fill attributes, a decorative model
dropdown that never affected its chart, dead code, an orphaned component,
and a broken CSS token reference.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/styles/tokens.css

@@ -70,13 +70,19 @@
   --radius-md: 6px;
   --radius-lg: 12px;
 
-  /* — Chart-specific colors — */
-  --chart-modeled:   #000a9bff;  /* dark indigo-blue (from wp_model_palette) */
-  --chart-frequentist: #858585bb; /* gray (from wp_model_palette) */
-  --chart-observed:  #A6271D;    /* red for observed data points */
+  /* — Race categorical palette (validated, see src/utils/colors.js) — */
+  --race-wh: #3D6FC4;
+  --race-bl: #C98A2A;
+  --race-hi: #9C3F86;
+  --race-am: #3D9A6B;
+
+  /* — Chart-specific colors — aliased onto the tokens charts actually use */
+  --chart-modeled:     var(--cv-navy-600);
+  --chart-frequentist: var(--cv-ink-4);
+  --chart-observed:    var(--cv-ink);
 
   /* — Model quadrant colors — */
-  --model-one-year-baseline:   var(--navy-600);
+  --model-one-year-baseline:   var(--cv-navy-600);
   --model-three-year-baseline: var(--teal-600);
 }