feat: make the edge-lit LCD the site's nav bar

nav-bar.css (after gantry.css) draws the bargraph, caret, indicator,
chapter drop and ripple, route glints, brake, self-test and shimmer in
LCD pixels under gantry's mask, and masks the wave to the lower bezel.
Rules hang off html:root, the same weight the preview attribute had, so
they still beat gantry.css.

useNavBarSignals mirrors the speedometer, drive controls and chapter
onto the bar, writing only on change; the per-frame speed pair goes on
the LCD and wave rather than the nav, which keeps the per-frame restyle
small. SiteNavigation renders just the parts the bar uses.

tests/unit/nav-bar-hue.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest';
+import { hueTier } from '@/composables/useNavBarSignals';
+
+describe('nav bar hue tier', () => {
+  it('comes on at 80, 88 and 95 % of the dial', () => {
+    expect(hueTier(0, 0.79)).toBe(0);
+    expect(hueTier(0, 0.8)).toBe(1);
+    expect(hueTier(0, 0.88)).toBe(2);
+    expect(hueTier(0, 1)).toBe(3);
+  });
+
+  it('holds a tier until the speed falls below its own floor', () => {
+    expect(hueTier(1, 0.77)).toBe(1);
+    expect(hueTier(1, 0.75)).toBe(0);
+    expect(hueTier(3, 0.93)).toBe(3);
+    expect(hueTier(3, 0.9)).toBe(2);
+    expect(hueTier(3, 0.4)).toBe(0);
+  });
+});