Animate sidebar sub-nav unfurl

Add sidebar sub-nav expand/collapse unfurl, now driven by the CSS View
Transitions API instead of JS tweening. Each nav section is a
view-transition group that clips into a top-to-bottom reveal as it opens
or closes, with a masked gradient soft edge.

A small JS flag (`sidebar-unfurling` on the document element) gates the
gradient to actual open/close, since CSS can't tell a within-section
navigation from a section toggle.

public/marketing/marketing.css

@@ -672,3 +672,67 @@
 ::view-transition-old(sidebar-dot):only-child {
   animation: sidebar-dot-out 0.2s ease both;
 }
+
+/* Marketing sidebar sub-nav unfurl (see Sidebar.svelte). Each nav section is a
+   view-transition group. As one section grows/shrinks, the sections below tween
+   their boxes to the new positions (smooth reflow, no jump), and clipping the
+   growing/shrinking box turns the height change into a top-to-bottom unfurl.
+   These always run, but on a same-section navigation the boxes don't change
+   size so it's a no-op. */
+::view-transition-group(install-section),
+::view-transition-group(learn-section),
+::view-transition-group(sidebar-external) {
+  animation-duration: 0.25s;
+  animation-timing-function: ease-out;
+}
+
+::view-transition-group(install-section),
+::view-transition-group(learn-section) {
+  overflow: clip;
+}
+
+/* Anchor the section snapshots to the top at natural height, so a growing/
+   shrinking group reveals/hides content by clipping rather than scaling it. */
+::view-transition-old(install-section),
+::view-transition-new(install-section),
+::view-transition-old(learn-section),
+::view-transition-new(learn-section) {
+  height: auto;
+  inset-block-start: 0;
+}
+
+/* Animatable soft-edge distance for the reveal, registered so it can tween. */
+@property --subnav-fade {
+  syntax: "<length>";
+  inherits: false;
+  initial-value: 0px;
+}
+
+@keyframes subnav-fade-edge {
+  /* Hold the soft edge at full through the 0.25s reveal (0-78% of this 0.32s),
+     then dissolve it over the short tail so the last item keeps its soft edge
+     while revealing and the gradient is gone before the pseudo is removed (no
+     pop, no hard cut). */
+  0%,
+  78% {
+    --subnav-fade: 2rem;
+  }
+  100% {
+    --subnav-fade: 0rem;
+  }
+}
+
+/* The soft-edge gradient only applies when a section actually opens or closes:
+   the sidebar adds `.sidebar-unfurling` to the document element for exactly
+   those navigations (see Sidebar.svelte), so moving *within* a section shows no
+   gradient. `max()` floors the opaque region to the header height so a
+   collapsing section never fades its own header text. */
+:root.sidebar-unfurling::view-transition-image-pair(install-section),
+:root.sidebar-unfurling::view-transition-image-pair(learn-section) {
+  mask-image: linear-gradient(
+    to bottom,
+    #000 max(1.75rem, calc(100% - var(--subnav-fade))),
+    transparent
+  );
+  animation: subnav-fade-edge 0.32s ease-out both;
+}

src/marketing/Sidebar.svelte

@@ -34,6 +34,27 @@
     ["/learn", "/principles", "/faq", "/glossary"].includes(pathname) ||
     pathname.startsWith("/guides");
 
+  // Flag the document element only when a section actually opens or closes, so
+  // the sub-nav unfurl gradient (see marketing.css) fires on expand/collapse
+  // but not when navigating within an already-open section.
+  let sidebarStateInitialised = false;
+  let unfurlTimeout: ReturnType<typeof setTimeout>;
+  $: flagSectionChange(installOpen, learnOpen);
+  function flagSectionChange(_installOpen: boolean, _learnOpen: boolean) {
+    if (!sidebarStateInitialised) {
+      sidebarStateInitialised = true;
+      return;
+    }
+    if (typeof document === "undefined") {
+      return;
+    }
+    document.documentElement.classList.add("sidebar-unfurling");
+    clearTimeout(unfurlTimeout);
+    unfurlTimeout = setTimeout(() => {
+      document.documentElement.classList.remove("sidebar-unfurling");
+    }, 500);
+  }
+
   function onNav(event: MouseEvent, route: Route) {
     if (useDefaultNavigation(event)) {
       return;
@@ -150,7 +171,7 @@
 
 <aside class="sidebar">
   <nav class="top-section">
-    <div class="nav-section">
+    <div class="nav-section" style="view-transition-name: install-section;">
       <a
         href={routeToPath(installRoute)}
         class="section-header"
@@ -201,7 +222,7 @@
       {/if}
     </div>
 
-    <div class="nav-section">
+    <div class="nav-section" style="view-transition-name: learn-section;">
       <a
         href={routeToPath(learnRoute)}
         class="section-header"
@@ -258,7 +279,7 @@
       {/if}
     </div>
 
-    <div class="external-links">
+    <div class="external-links" style="view-transition-name: sidebar-external;">
       <a
         href={routeToPath(exploreRoute)}
         class="external-link arrow-link"