feat(story): sticky nav title + floating back-to-top pill

Nav title: absolutely centered in the site-header, fades in via
IntersectionObserver when .story-hero__content scrolls above the fold.
Hidden on mobile (< 640px) where there is no room.

Back-to-top: fixed bottom-right pill, appears after 80% of the hero
viewport is scrolled past, smooth-scrolls to top on click.

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

themes/intotheeast/css/style.css

@@ -1194,6 +1194,52 @@ body::after {
     text-decoration: none;
 }
 
+/* ── Story nav title (desktop: fades into header center) ─────── */
+.story-nav-title {
+    position: absolute;
+    left: 50%;
+    top: 50%;
+    transform: translateX(-50%) translateY(-50%);
+    font-family: var(--font-ui);
+    font-size: var(--text-sm);
+    font-weight: 500;
+    color: var(--color-ink);
+    white-space: nowrap;
+    max-width: 40%;
+    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 ─────────────────────────────────────── */
+.story-totop {
+    position: fixed;
+    bottom: 1.5rem;
+    right: 1.5rem;
+    font-family: var(--font-ui);
+    font-size: var(--text-xs);
+    font-weight: 500;
+    letter-spacing: 0.04em;
+    color: var(--color-ink);
+    background: var(--color-canvas);
+    border: 1px solid var(--color-border);
+    border-radius: 9999px;
+    padding: 0.4rem 0.9rem;
+    cursor: pointer;
+    opacity: 0;
+    transform: translateY(6px);
+    transition: opacity 0.3s ease, transform 0.3s ease, background 0.15s;
+    pointer-events: none;
+    z-index: 90;
+    box-shadow: 0 2px 8px rgba(0,0,0,0.08);
+}
+.story-totop.is-visible { opacity: 1; transform: translateY(0); pointer-events: auto; }
+.story-totop:hover { background: var(--color-paper); }
+
 /* ── ChapterBreak ─────────────────────────────────────────── */
 .chapter-break {
     position: relative;