feat: add SPA navigation with view transitions

Swap page content over the existing SSE connection without full page
loads. A persistent Context resets its page-specific state (signals,
actions, intervals, subscriptions) on navigate while preserving the
SSE stream, CSRF token, and session.

- c.Navigate(path) for programmatic SPA navigation from actions
- Injected JS intercepts same-origin <a> clicks (opt out with
  data-via-no-boost) and handles popstate for back/forward
- v.Layout() wraps pages in a shared shell for DRY nav/chrome
- View Transition API integration via WithViewTransitions() on
  PatchElements and h.DataViewTransition() helper
- POST /_navigate endpoint with CSRF validation and rate limiting
- pageStopChan cancels page-level OnInterval goroutines on navigate
- Includes SPA example with layout, counter, and live clock pages

navigate.js

@@ -0,0 +1,52 @@
+(function() {
+  const meta = document.querySelector('meta[data-signals]');
+  if (!meta) return;
+  const raw = meta.getAttribute('data-signals');
+  const parsed = JSON.parse(raw.replace(/'/g, '"'));
+  const ctxID = parsed['via-ctx'];
+  const csrf = parsed['via-csrf'];
+  if (!ctxID || !csrf) return;
+
+  function navigate(url, popstate) {
+    const params = new URLSearchParams({
+      'via-ctx': ctxID,
+      'via-csrf': csrf,
+      'url': url,
+    });
+    if (popstate) params.set('popstate', '1');
+    fetch('/_navigate', {
+      method: 'POST',
+      headers: {'Content-Type': 'application/x-www-form-urlencoded'},
+      body: params.toString()
+    }).then(function(res) {
+      if (!res.ok) window.location.href = url;
+    }).catch(function() {
+      window.location.href = url;
+    });
+  }
+
+  document.addEventListener('click', function(e) {
+    var el = e.target;
+    while (el && el.tagName !== 'A') el = el.parentElement;
+    if (!el) return;
+    if (e.ctrlKey || e.metaKey || e.shiftKey || e.altKey) return;
+    if (el.hasAttribute('target')) return;
+    if (el.hasAttribute('data-via-no-boost')) return;
+    var href = el.getAttribute('href');
+    if (!href || href.startsWith('#')) return;
+    try {
+      var url = new URL(href, window.location.origin);
+      if (url.origin !== window.location.origin) return;
+      e.preventDefault();
+      navigate(url.pathname + url.search);
+    } catch(_) {}
+  });
+
+  var ready = false;
+  window.addEventListener('popstate', function() {
+    if (!ready) { ready = true; return; }
+    navigate(window.location.pathname + window.location.search, true);
+  });
+  // Mark as ready after initial load completes
+  setTimeout(function() { ready = true; }, 0);
+})();