fix(story): nav title cross-fades scroll-driven as hero content exits viewport

Replaced IntersectionObserver (discrete threshold) with a scroll RAF loop
using getBoundingClientRect. Opacity is computed from the fraction of
.story-hero__content still visible above the viewport top — so the nav title
fades in gradually as the hero title slides off the top edge, reaching full
opacity only when the element is completely gone.

Removed CSS transition (no longer needed; per-frame JS update is smooth).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

themes/intotheeast/css/style.css

@@ -1210,10 +1210,8 @@ body::after {
     overflow: hidden;
     text-overflow: ellipsis;
     opacity: 0;
-    transition: opacity 0.35s ease;
     pointer-events: none;
 }
-.story-nav-title.is-visible { opacity: 1; }
 @media (max-width: 640px) { .story-nav-title { display: none; } }
 
 /* ── Back to top pill ─────────────────────────────────────── */