feat: three-tier context lifecycle (grace → suspended → reaped)

Contexts that lose their SSE connection now pass through a suspended
state before being fully reaped. Suspended contexts keep their shell
(ID, route, CSRF token) but free page resources. On reconnect, the
page init function is re-run for a seamless resume. Contexts past
the TTL trigger a client-side reload instead of a silent dead page.

Configurable via ContextSuspendAfter (default 15m) and ContextTTL
(default 1h).

context.go

@@ -42,6 +42,7 @@ type Context struct {
 	createdAt           time.Time
 	sseConnected        atomic.Bool
 	sseDisconnectedAt   atomic.Pointer[time.Time]
+	suspended           atomic.Bool
 }
 
 // View defines the UI rendered by this context.
@@ -400,6 +401,13 @@ func (c *Context) resetPageState() {
 	c.mu.Unlock()
 }
 
+// suspend frees page-scoped resources while keeping the context shell alive
+// in the registry for seamless re-init on reconnect.
+func (c *Context) suspend() {
+	c.resetPageState()
+	c.suspended.Store(true)
+}
+
 // Navigate performs an SPA navigation to the given path. It resets page state,
 // runs the target page's init function (with middleware), and pushes the new
 // view over the existing SSE connection with a view transition animation.