feat(route): single right-edge advance arrow — forward-only affordance

Replaces the prev/next button pair removed last commit with a single
viewport-anchored circular control.

  - 48px diameter, 1px terracotta border, terracotta chevron on cream.
  - Hover / focus-visible fills to terracotta with cream chevron and a
    1.06× scale-up.
  - Anchored absolute inside .rr-wrap (already position: relative):
    right: 32px / top: 50% / translateY(-50%).
  - Toggles .rr-at-end (opacity 0.25, pointer-events: none) when the
    scroll container reaches its right edge.
  - First-load hint: .rr-hint class added 100ms after mount fires a
    rr-advance-pulse keyframe three times (iteration-count: 3) — soft
    8px shadow ring in 15% terracotta pulses out and back. Animation
    stops naturally; no JS cleanup needed.

No left arrow on purpose — the path reads past → future, and the
user's instinct at any milestone is 'what's next?' The right arrow
earns its keep by hinting the existence of more track beyond the
visible window. A symmetric left arrow would just be noise.

Click handler today: scrollBy({behavior: 'smooth'}) by 60% of viewport
width. Step 4 replaces this with a custom-animated glide and adds the
drag + wheel scroll modalities.

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

src/components/RoadmapRoute.astro

@@ -107,6 +107,21 @@ const initialShippingX = lastShippingIndex >= 0 ? layout.itemX[lastShippingIndex
 
     <div class="rr-fade-left" id="rr-fade-l" aria-hidden="true"></div>
     <div class="rr-fade-right" id="rr-fade-r" aria-hidden="true"></div>
+
+    <!-- Single forward-only advance affordance anchored to the right
+         viewport edge. There's no left arrow on purpose — the path
+         reads left-to-right and the user's instinct after looking at
+         a milestone is 'what's next?', not 'what came before?'. -->
+    <button
+      type="button"
+      class="rr-advance"
+      id="rr-advance"
+      aria-label="Further along the route"
+    >
+      <svg viewBox="0 0 24 24" width="14" height="14" aria-hidden="true">
+        <path d="M9 6l6 6-6 6" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+      </svg>
+    </button>
   </div>
 
   <!-- Legend lives in /roadmap.astro now so it returns to centred
@@ -153,6 +168,7 @@ const initialShippingX = lastShippingIndex >= 0 ? layout.itemX[lastShippingIndex
     const milestones = Array.from(section.querySelectorAll<HTMLElement>('.rr-milestone'));
     const fadeL  = section.querySelector<HTMLElement>('#rr-fade-l');
     const fadeR  = section.querySelector<HTMLElement>('#rr-fade-r');
+    const advance = section.querySelector<HTMLButtonElement>('#rr-advance');
     if (!scroll || !track || !svg) return;
 
     const itemCount = milestones.length;
@@ -194,16 +210,22 @@ const initialShippingX = lastShippingIndex >= 0 ? layout.itemX[lastShippingIndex
       milestones.forEach((m, i) => { m.style.left = `${itemX[i]}px`; });
     }
 
-    /* Arrow handler + edge-state updates are rewritten in the next commit
-       (drag + wheel + glide). For now only the fades update on scroll. */
+    /* Edge state — fades + advance disable. The smooth glide animation
+       on advance click lands in the next commit; for now the click
+       uses native smooth scrollBy. */
     function updateNav() {
       const max = scroll!.scrollWidth - scroll!.clientWidth;
       const atStart = scroll!.scrollLeft <= 2;
       const atEnd   = scroll!.scrollLeft >= max - 2;
       if (fadeL) fadeL.style.opacity = atStart ? '0' : '1';
       if (fadeR) fadeR.style.opacity = atEnd ? '0' : '1';
+      if (advance) advance.classList.toggle('rr-at-end', atEnd);
     }
 
+    advance?.addEventListener('click', () => {
+      scroll!.scrollBy({ left: scroll!.clientWidth * 0.6, behavior: 'smooth' });
+    });
+
     scroll.addEventListener('scroll', updateNav, { passive: true });
 
     // Debounced resize → recompute layout + refresh nav state. 120ms is
@@ -226,6 +248,10 @@ const initialShippingX = lastShippingIndex >= 0 ? layout.itemX[lastShippingIndex
     }
     setTimeout(updateNav, 50);
     updateNav();
+
+    // Three-pulse hint on the advance arrow ~100ms after layout settles
+    // so the user notices the affordance once and then it sits quietly.
+    setTimeout(() => advance?.classList.add('rr-hint'), 100);
   });
 </script>
 
@@ -387,6 +413,50 @@ const initialShippingX = lastShippingIndex >= 0 ? layout.itemX[lastShippingIndex
     margin: 0;
   }
 
+  /* ── Advance arrow ─────────────────────────────────────────────── */
+  .rr-advance {
+    position: absolute;
+    right: 32px;
+    top: 50%;
+    transform: translateY(-50%);
+    width: 48px;
+    height: 48px;
+    border-radius: 50%;
+    border: 1px solid var(--pigment-terracotta);
+    background: var(--background);
+    color: var(--pigment-terracotta);
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 0;
+    z-index: 5;
+    transition: background .2s ease,
+                color      .2s ease,
+                opacity    .25s ease,
+                transform  .25s ease;
+  }
+  .rr-advance:hover,
+  .rr-advance:focus-visible {
+    background: var(--pigment-terracotta);
+    color: var(--background);
+    outline: none;
+    transform: translateY(-50%) scale(1.06);
+  }
+  .rr-advance[disabled],
+  .rr-advance.rr-at-end {
+    opacity: 0.25;
+    pointer-events: none;
+  }
+  /* Three-pulse hint on first load — fires once, then stops. */
+  @keyframes rr-advance-pulse {
+    0%, 100% { box-shadow: 0 0 0 0   rgba(185, 107, 88, 0); }
+    50%      { box-shadow: 0 0 0 8px rgba(185, 107, 88, 0.15); }
+  }
+  .rr-advance.rr-hint {
+    animation: rr-advance-pulse 1.4s ease-in-out 3;
+  }
+
   /* Edge fades cover only the track itself — the top/bottom padding
      zones (60/80) on .rr-scroll exist so hover cards can overflow there
      without clipping, so the fades shouldn't paint over them. */