Address review: a done target stays off-limits for a replay

Marking a target done on reply opened a window: an EventSource reconnect
replays the still-pending target while the result is on the wire, the
tab is GENERATING by then, so it declined busy, the server handed the
lease back mid-resolution, and another tab could claim and fire a second
Go. `agentTargetTaken` now covers both acting and done, so a replay of a
target this page took a lease on is ignored, and the late-mount watch
stops on either. Contract pins for the guard and the watch.

Written with AI assistance (Claude).

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

skill/scripts/live-browser.js

@@ -7289,18 +7289,29 @@
   }
 
   // This page's participation in each target it heard: 'acting' once a
-  // claim was granted (so a replay never starts a second Go), else the word
-  // it last gave. The server replays pending targets to every connection
-  // that opens. After a reconnect that overlapped the old connection the
-  // server still holds this page's word; after one that did not, it dropped
-  // the word on the close, so a replayed target is handled again: a busy or
-  // unresolvable page re-declines (idempotent), an idle page claims.
+  // claim was granted, 'done' once it replied (or stood down from a lapsed
+  // lease), else the word it last gave. The server replays pending targets
+  // to every connection that opens. After a reconnect that overlapped the
+  // old connection the server still holds this page's word; after one that
+  // did not, it dropped the word on the close, so a replayed target is
+  // handled again: a busy or unresolvable page re-declines (idempotent), an
+  // idle page claims.
   const agentTargetsSeen = new Map();
   function noteAgentTarget(targetId, status) {
     agentTargetsSeen.set(targetId, status);
     if (agentTargetsSeen.size > 100) agentTargetsSeen.delete(agentTargetsSeen.keys().next().value);
   }
 
+  // A target this page took a lease on is off-limits for a replay: while
+  // acting (a second claim or Go), and once done, because its result may
+  // still be on the wire and this tab is GENERATING by then, so handling
+  // the replay would decline busy, hand the lease back mid-resolution, and
+  // let another tab fire a second Go.
+  function agentTargetTaken(targetId) {
+    const status = agentTargetsSeen.get(targetId);
+    return status === 'acting' || status === 'done';
+  }
+
   // Only a page that can resolve the target claims it. A tab whose page
   // lacks the element declines with its resolution verdict instead, so a
   // first-wins claim never lets the wrong page answer for a target that
@@ -7338,7 +7349,7 @@
   }
 
   function watchAgentTargetResolution(msg, lastError) {
-    if (agentTargetOverlayGone() || agentTargetsSeen.get(msg.targetId) === 'acting') return;
+    if (agentTargetOverlayGone() || agentTargetTaken(msg.targetId)) return;
     const busy = agentTargetBusyReason(msg.targetId);
     if (busy) { declineAgentTargetBusy(msg, busy); return; }
     const probe = resolveAgentTargetElement(msg);
@@ -7350,7 +7361,7 @@
 
   function handleAgentTarget(msg) {
     if (!msg || typeof msg.targetId !== 'string') return;
-    if (agentTargetsSeen.get(msg.targetId) === 'acting') return;
+    if (agentTargetTaken(msg.targetId)) return;
     noteAgentTarget(msg.targetId, 'heard');
     const busy = agentTargetBusyReason(msg.targetId);
     if (busy) {

tests/live-browser-source.test.mjs

@@ -852,8 +852,18 @@ describe('live-browser source contracts', () => {
     );
     assert.match(
       SOURCE,
-      /function handleAgentTarget\(msg\) \{[\s\S]{0,120}?if \(agentTargetsSeen\.get\(msg\.targetId\) === 'acting'\) return;/,
-      'a replayed target this page is acting on must not start a second claim or Go; any other replay is handled again',
+      /function handleAgentTarget\(msg\) \{[\s\S]{0,120}?if \(agentTargetTaken\(msg\.targetId\)\) return;/,
+      'a replayed target this page took a lease on must not start a second claim, Go, or decline; any other replay is handled again',
+    );
+    assert.match(
+      SOURCE,
+      /function agentTargetTaken\(targetId\) \{[\s\S]{0,200}?status === 'acting' \|\| status === 'done';/,
+      'a done target is still taken: a replay while its result is on the wire must not decline busy and hand the lease to a second Go',
+    );
+    assert.match(
+      SOURCE,
+      /function watchAgentTargetResolution\(msg, lastError\) \{\s*if \(agentTargetOverlayGone\(\) \|\| agentTargetTaken\(msg\.targetId\)\) return;/,
+      'the late-mount watch stops once this page took the lease',
     );
     assert.match(
       SOURCE,