CB-WP-0035: a session that answers nothing

Two reports, one cause: the script's fetch had no .catch. When the server
had exited the promise rejected, the chain never ran, and not even the
status line moved — so a dead server and a click that did nothing were
indistinguishable. play again read as a dead button, and the linger
timeout was invisible.

Now a rejection says the session is gone and seals the page, and a
5-second heartbeat against a new /alive notices it without needing a
click, which is what the timeout case requires. The beat carries the
token, and does not extend the linger: that deadline is absolute.

The harness had the same hole. jsrun's fetch stub had no .catch, so the
branch that notices a dead server would have been unreachable in every
test — the very defect the stub's own comment records from CB-WP-0024.
Teaching it __failing, .catch and a recorded setInterval was the fix;
writing the script defensively would have repeated the trap.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/cb-render-html/src/doc.rs

@@ -91,7 +91,9 @@ pub const SCRIPT: &str = r#"
   //
   // No game vocabulary here (ADR-0007 D5). 'closed' is a server
   // lifecycle word, exactly like the 'ok' branch below it.
+  var sealed = false;
   function seal() {
+    sealed = true;
     // The WHOLE page goes inert, not only the controls. A greyed-out
     // button beside a full-colour table still reads as a live game with
     // one broken control; the session has ended and everything on screen
@@ -206,8 +208,33 @@ pub const SCRIPT: &str = r#"
       status(t);
       if (t.indexOf('ok') === 0) { window.location.reload(); }
       else if (t.indexOf('closed') === 0) { seal(); }
-    });
+    }).catch(gone);
   });
+
+  // CB-WP-0035. The session ended without this page being told.
+  //
+  // There was no rejection handler at all, so when the server had exited
+  // the promise rejected and the chain simply never ran -- not even the
+  // status line moved. `play again` looked like a dead button, and the
+  // linger timeout was invisible. A request that cannot be answered is
+  // information, and it was being thrown away.
+  function gone() {
+    status('the session is no longer available \u2014 the game server has '
+         + 'stopped. Nothing on this page can act; it is a record now.');
+    seal();
+  }
+
+  // And notice it WITHOUT being clicked, which is what the timeout needs:
+  // a player who walks away comes back to a sealed page rather than to a
+  // live-looking table that answers nothing.
+  if (window.CB_ALIVE) {
+    setInterval(function () {
+      if (sealed) { return; }
+      fetch(window.CB_ALIVE).then(function (r) {
+        if (!r.ok) { gone(); }
+      }).catch(gone);
+    }, 5000);
+  }
 })();
 "#;
 
@@ -949,6 +976,7 @@ pub fn document(
         Endpoints {
             command: endpoint,
             note: "/note",
+            alive: "/alive",
         },
         seat,
         may_pass,
@@ -969,6 +997,8 @@ pub struct Endpoints<'a> {
     pub command: &'a str,
     /// Free text. Nothing turns these into commands.
     pub note: &'a str,
+    /// Where the page checks the session is still there (CB-WP-0035).
+    pub alive: &'a str,
 }
 
 pub fn document_with_log(
@@ -980,7 +1010,7 @@ pub fn document_with_log(
     log: Account<'_>,
     meta: &[String],
 ) -> String {
-    let (endpoint, note_to) = (to.command, to.note);
+    let (endpoint, note_to, alive) = (to.command, to.note, to.alive);
     let mut s = String::with_capacity(8192);
     let _ = write!(
         s,
@@ -1019,9 +1049,10 @@ pub fn document_with_log(
     let _ = write!(
         s,
         "<div id=\"cb-status\">ready</div>\
-         <script>window.CB_ENDPOINT={endpoint}</script><script>{SCRIPT}</script>\
-         </body></html>",
+         <script>window.CB_ENDPOINT={endpoint};window.CB_ALIVE={alive}</script>\
+         <script>{SCRIPT}</script></body></html>",
         endpoint = json_string(endpoint),
+        alive = json_string(alive),
     );
     s
 }
@@ -1733,6 +1764,7 @@ pub fn ending(
     message: &str,
     endpoint: &str,
     note_to: Option<&str>,
+    alive: &str,
     log: Account<'_>,
     series: &[String],
 ) -> String {
@@ -1814,8 +1846,10 @@ pub fn ending(
     let _ = write!(
         s,
         "<div id=\"cb-status\">the game is over</div>\
-         <script>window.CB_ENDPOINT={endpoint}</script><script>{SCRIPT}</script>",
+         <script>window.CB_ENDPOINT={endpoint};window.CB_ALIVE={alive}</script>\
+         <script>{SCRIPT}</script>",
         endpoint = json_string(endpoint),
+        alive = json_string(alive),
     );
     s
 }