wip: Campaign LA LA2 probe mode + idle policy — INCOMPLETE, stopped mid-task

Agent was stopped for token budget. Landed here: probe flag through
LiveSessionConnectOptions + the StartCore short-circuit, the mode field
with JsonRequired-to-semantic-validation move, host exit-code mapping,
and 34 passing tests including 3 new probe tests (agent last reported
green before the stop). NOT DONE: the idle-policy unit tests (next
step), full-suite verification, and the WSL run.

Build/test state UNVERIFIED at this commit. Next session: finish idle
policy tests, run Runtime+Headless Release suites Windows and WSL, then
dispatch the Opus dual-lens review.

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

tests/AcDream.Runtime.Tests/Session/LiveSessionControllerTests.cs

@@ -389,6 +389,81 @@ public sealed class LiveSessionControllerTests
         Assert.Equal(1, operations.DisposeCounts[operations.Sessions[0]]);
     }
 
+    /// <summary>
+    /// Campaign LA slice LA2: the probe short-circuit — connect, receive
+    /// CharacterList, report the roster, then gracefully disconnect via the
+    /// SAME StopCore teardown <see cref="Start_NoAvailableCharacterTearsDownExactScope"/>
+    /// exercises, returning <see cref="LiveSessionStartStatus.ProbeComplete"/>
+    /// instead of ever reaching TrySelectCharacter/ApplySelectedCharacter/
+    /// EnterWorld. Asserted directly against the operations fake:
+    /// <see cref="TestOperations.EnterWorldCount"/> stays zero and "enter:*"/
+    /// "selected"/"activate"/"entered" never appear in the call trace.
+    /// </summary>
+    [Fact]
+    public void Start_ProbeReportsRosterThenGracefullyDisconnectsWithoutSelectionOrEnterWorld()
+    {
+        var calls = new List<string>();
+        var operations = new TestOperations(calls);
+        var host = new TestHost(calls);
+        var controller = new LiveSessionController(operations);
+
+        LiveSessionStartResult result = controller.Start(
+            LiveOptions(probe: true),
+            host);
+
+        Assert.Equal(LiveSessionStartStatus.ProbeComplete, result.Status);
+        Assert.Null(result.Selection);
+        Assert.Equal(
+            [
+                "reset", "resolve", "create", "bind", "report-connecting",
+                "connect", "report-connected", "roster", "deactivate",
+                "detach-events", "dispose-session", "detach-session", "reset",
+            ],
+            calls);
+        Assert.DoesNotContain("selected", calls);
+        Assert.DoesNotContain("activate", calls);
+        Assert.DoesNotContain("entered", calls);
+        Assert.Equal(0, operations.EnterWorldCount);
+        LiveSessionRosterReport roster = Assert.Single(host.Rosters);
+        Assert.Equal("Canonical", roster.AccountName);
+        Assert.False(controller.IsInWorld);
+        Assert.Null(controller.CurrentSession);
+        Assert.Equal(1, operations.DisposeCounts[operations.Sessions[0]]);
+
+        // The same 4-stage graceful teardown the NoCharacters path uses —
+        // the probe's scope fully converges without requiring
+        // controller.Dispose().
+        LiveSessionOwnershipSnapshot ownership = controller.CaptureOwnership();
+        Assert.Equal(RuntimeTeardownStage.Complete, ownership.LastTeardownStages);
+        Assert.False(ownership.HasActiveSession);
+        Assert.False(ownership.HasRetiredSession);
+        Assert.False(ownership.HasPendingOperation);
+    }
+
+    /// <summary>
+    /// The probe short-circuit fires even when the server never returns a
+    /// CharacterList at all (GetCharacters returns null) — a probe is a
+    /// connectivity check, not itself a character-selection operation, so it
+    /// must not fall through to the NoCharacters path.
+    /// </summary>
+    [Fact]
+    public void Start_ProbeWithoutCharacterListStillCompletesGracefully()
+    {
+        var calls = new List<string>();
+        var operations = new TestOperations(calls) { Characters = null };
+        var host = new TestHost(calls);
+        var controller = new LiveSessionController(operations);
+
+        LiveSessionStartResult result = controller.Start(
+            LiveOptions(probe: true),
+            host);
+
+        Assert.Equal(LiveSessionStartStatus.ProbeComplete, result.Status);
+        Assert.Empty(host.Rosters);
+        Assert.Equal(0, operations.EnterWorldCount);
+        Assert.False(controller.IsInWorld);
+    }
+
     [Theory]
     [InlineData("index")]
     [InlineData("id")]
@@ -1141,14 +1216,16 @@ public sealed class LiveSessionControllerTests
     private static LiveSessionConnectOptions LiveOptions(
         bool live = true,
         string? user = "user",
-        LiveSessionCharacterSelector? selector = null) =>
+        LiveSessionCharacterSelector? selector = null,
+        bool probe = false) =>
         new(
             live,
             "127.0.0.1",
             9000,
             user ?? string.Empty,
             "password",
-            selector);
+            selector,
+            probe);
 
     private static CharacterList.Parsed AvailableCharacters() => new(
         0u,