coalesce concurrent session spawns onto one future (T-374)

Orchestrator.spawn() check-then-acts on the session registry across
two awaits (transcript-tail read, process start) — two racing callers
for the same id both passed the check and the loser's live claude
process was orphaned, never killed, never observed. The first caller
now installs the spawn future synchronously; later callers await the
same future, and a failed spawn clears the entry so a retry proceeds.

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

test/builtin/claude/session_orchestrator_test.dart

@@ -60,6 +60,32 @@ void main() {
     expect(created, hasLength(1));
   });
 
+  // T-374: spawn() check-then-acts across awaits; without the in-flight
+  // map, two CONCURRENT spawns both passed the registry check and the
+  // loser's live claude process was orphaned.
+  test('two concurrent spawns for one id share one session and one process (T-374)', () async {
+    final (a, b) = await (orch.spawn(spec('primary')), orch.spawn(spec('primary'))).wait;
+    expect(identical(a, b), isTrue);
+    expect(created, hasLength(1));
+  });
+
+  test('a failed spawn clears the in-flight entry so a retry can proceed (T-374)', () async {
+    var calls = 0;
+    final flaky = ClaudeSessionOrchestrator(
+      processFactory: ({required sessionArgs, required cwd, env}) async {
+        calls++;
+        if (calls == 1) throw StateError('spawn blew up');
+        final p = _FakeProc();
+        created.add(p);
+        return p;
+      },
+    );
+    await expectLater(flaky.spawn(spec('primary')), throwsStateError);
+    final m = await flaky.spawn(spec('primary'));
+    expect(m.id, 'primary');
+    expect(calls, 2);
+  });
+
   test('hide keeps the process alive and in the registry; show restores it', () async {
     await orch.spawn(spec('primary'));
     orch.hide('primary');