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

routine.go

@@ -5,7 +5,7 @@ import (
 	"time"
 )
 
-func newOnInterval(ctxDisposedChan chan struct{}, duration time.Duration, handler func()) func() {
+func newOnInterval(ctxDisposedChan, pageStopChan chan struct{}, duration time.Duration, handler func()) func() {
 	localInterrupt := make(chan struct{})
 	var stopped atomic.Bool
 
@@ -16,6 +16,8 @@ func newOnInterval(ctxDisposedChan chan struct{}, duration time.Duration, handle
 			select {
 			case <-ctxDisposedChan:
 				return
+			case <-pageStopChan:
+				return
 			case <-localInterrupt:
 				return
 			case <-tkr.C: