feat(route): nav script — arrows, fades, progress dots, initial scroll

Vanilla TS script at the bottom of RoadmapRoute.astro. No library.

- Arrows scrollBy ±72% of the scroll-container's clientWidth, smooth
  behaviour. Disabled at scroll start/end.
- Edge fades (.rr-fade-left / -right) flip opacity 0↔1 at scroll start /
  end so the affordance disappears when there's nowhere further to go.
- Progress dots track scrollLeft/(scrollWidth-clientWidth) percentage,
  bucketing into dots.length slots. Active dot gets .active (themed in
  CSS as --on-surface).
- On mount, the script reads section.data-initial-x — the SVG x position
  of the most recent shipping milestone (computed server-side from the
  layout helper) — and scrolls so that x sits ~25% from the left edge
  of the viewport. Clamped to [0, scrollWidth-clientWidth]. Member opens
  /roadmap and immediately sees one shipped item + several ahead-of-them
  items, not the very start of history.
- setTimeout(update, 50) re-measures after first paint settles
  (especially relevant when SVG fonts or other late-arriving assets
  shift the trackWidth by a couple of px).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/RoadmapRoute.astro

@@ -159,6 +159,58 @@ const initialShippingX = lastShippingIndex >= 0 ? itemXByIndex[lastShippingIndex
   </ol>
 </section>
 
+<script>
+  // Vanilla nav for the desktop horizontal route:
+  //  - arrows scrollBy 72% of the viewport per click
+  //  - edge fades flip on at-start / at-end
+  //  - progress dots track scroll position
+  //  - on mount, scroll the 'you are here' milestone roughly 25% from left
+  document.querySelectorAll<HTMLElement>('.route').forEach((section) => {
+    const scroll = section.querySelector<HTMLElement>('#rr-scroll');
+    const prev   = section.querySelector<HTMLButtonElement>('#rr-prev');
+    const next   = section.querySelector<HTMLButtonElement>('#rr-next');
+    const fadeL  = section.querySelector<HTMLElement>('#rr-fade-l');
+    const fadeR  = section.querySelector<HTMLElement>('#rr-fade-r');
+    const dots   = Array.from(section.querySelectorAll<HTMLElement>('#rr-progress .rr-progress-dot'));
+    if (!scroll) return;
+
+    const step = () => scroll.clientWidth * 0.72;
+
+    prev?.addEventListener('click', () => scroll.scrollBy({ left: -step(), behavior: 'smooth' }));
+    next?.addEventListener('click', () => scroll.scrollBy({ left:  step(), behavior: 'smooth' }));
+
+    function update() {
+      const max = scroll!.scrollWidth - scroll!.clientWidth;
+      const atStart = scroll!.scrollLeft <= 2;
+      const atEnd   = scroll!.scrollLeft >= max - 2;
+      if (prev)  prev.disabled = atStart;
+      if (next)  next.disabled = atEnd;
+      if (fadeL) fadeL.style.opacity = atStart ? '0' : '1';
+      if (fadeR) fadeR.style.opacity = atEnd ? '0' : '1';
+      if (dots.length > 0) {
+        const pct = max > 0 ? scroll!.scrollLeft / max : 0;
+        const activeIdx = Math.min(dots.length - 1, Math.floor(pct * dots.length));
+        dots.forEach((d, i) => d.classList.toggle('active', i === activeIdx));
+      }
+    }
+
+    scroll.addEventListener('scroll', update, { passive: true });
+    window.addEventListener('resize', update);
+
+    // Initial scroll: park the most recent shipping item ~25% from the left.
+    const initialX = Number(section.dataset.initialX ?? 0);
+    if (initialX > 0) {
+      const max = scroll.scrollWidth - scroll.clientWidth;
+      const target = Math.max(0, Math.min(max, initialX - scroll.clientWidth * 0.25));
+      scroll.scrollLeft = target;
+    }
+
+    // First paint may happen before layout settles — re-measure shortly after.
+    setTimeout(update, 50);
+    update();
+  });
+</script>
+
 <style>
   /* ── Section header ─────────────────────────────────────────────── */
   .route-header {