docs(ui): drop sidebar saved-state — server defaults already ARE the standard docs behavior

Simplification per spec: the menu tracks state through the URL alone.
Land on /docs: fully closed, headers only (an explicit home rule now
overrides the contains-URL rule, since the root overview doc lives in
Getting Started and would otherwise auto-open it). Land on any doc page:
its containing section + group render open at first paint, current link
bold — no JS, no jump, arrows consistent. Navigate elsewhere: old
closes, new opens.

Deleted: docs_nav cookie const/parse, ApplyNavOverrides, cookie write +
collect/persist helpers in web/src/docs.ts, data-key attributes in the
sidebar templ, and the first-section-open fallback (docs home used to
force-open the first section — spec'd as closed).

toggleCollapsible remains for in-page peeking; purely ephemeral.
The [data-collapsed=true] CSS from the previous round stays — it is what
makes the server-rendered state actually hide sections at paint (the
original bug was that this CSS never existed).

Tests: TestBuildNavigationNothingCurrent now asserts zero sections open
on unmatched paths; TestBuildNavigation asserts Getting Started closed
on a device page (no site-wide pin). make test green; 146 vitests; live
verified: home 0 expanded, doc pages exactly 2 (section+group).

web/src/docs.ts

@@ -15,39 +15,13 @@ function toggleSidebar(): void {
   overlay.classList.toggle("hidden");
 }
 
-// Docs sidebar collapse persistence. Rendering ownership: the server
-// renders what is expanded (navigation defaults + docs_nav cookie
-// overrides merged in internal/docs), so first paint is already correct —
-// no post-paint correction, no jumping. This module only records intent:
-// every toggle rewrites the cookie with the full current state.
-
-const DOCS_NAV_COOKIE = "docs_nav";
-
-type DocsNavState = Record<string, boolean>;
-
-// collectDocsNavState reads the current collapsed flag of every
-// collapsible in the sidebar straight from the DOM.
-function collectDocsNavState(): DocsNavState {
-  const state: DocsNavState = {};
-  document.querySelectorAll<HTMLElement>("[data-key][data-collapsed]").forEach((el) => {
-    const key = el.getAttribute("data-key");
-    if (key) state[key] = el.getAttribute("data-collapsed") === "true";
-  });
-  return state;
-}
-
-function persistDocsNavCookie(state: DocsNavState): void {
-  try {
-    // Strictly-necessary UI state: first-party, no identifiers, one year.
-    document.cookie = `${DOCS_NAV_COOKIE}=${encodeURIComponent(JSON.stringify(state))}; path=/; max-age=31536000; SameSite=Lax`;
-  } catch {
-    // Storage blocked: the sidebar still works, state just doesn't persist.
-  }
-}
+// Docs sidebar sections toggle open/closed in-page (purely client-local,
+// ephemeral — the SERVER decides what is expanded on each page load from
+// navigation defaults, so there is no saved state and nothing to jump).
+// The data-collapsed attribute is the single source of truth: the
+// stylesheet hides on it, and the arrow rotates relative to it.
 
 function applyCollapsed(items: HTMLElement, collapsed: boolean): void {
-  // The data-collapsed attribute is the single source of truth — the
-  // stylesheet hides on it, and the arrow is rotated relative to it.
   items.setAttribute("data-collapsed", collapsed ? "true" : "false");
   const arrow = items.previousElementSibling?.querySelector<HTMLElement>(".section-arrow");
   if (arrow) {
@@ -60,9 +34,7 @@ function toggleCollapsible(header: HTMLElement): void {
   if (!items) return;
 
   const isCollapsed = items.getAttribute("data-collapsed") === "true";
-
   applyCollapsed(items, !isCollapsed);
-  persistDocsNavCookie(collectDocsNavState());
 }
 
 let docsSearchTimeout: ReturnType<typeof setTimeout> | null = null;