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>

src/AcDream.Runtime/Session/LiveSessionContracts.cs

@@ -13,7 +13,23 @@ public sealed record LiveSessionConnectOptions(
     int Port,
     string User,
     string Password,
-    LiveSessionCharacterSelector? Character = null);
+    LiveSessionCharacterSelector? Character = null,
+    /// <summary>
+    /// Campaign LA slice LA2: short-circuits
+    /// <see cref="LiveSessionController"/>'s connect transaction right after
+    /// the roster report (before <c>TrySelectCharacter</c>/
+    /// <c>ApplySelectedCharacter</c>/<c>EnterWorld</c>) — connect, receive
+    /// <c>CharacterList</c>, report the roster, gracefully disconnect via the
+    /// same <c>StopCore</c> teardown the <see cref="LiveSessionStartStatus.NoCharacters"/>
+    /// path already uses, and return
+    /// <see cref="LiveSessionStartStatus.ProbeComplete"/>. The pinned launch
+    /// contract (<c>docs/plans/2026-08-14-launcher-campaign.md</c> LA1's
+    /// <c>mode</c> field) requires a probe session to omit both
+    /// <see cref="Character"/> and its policy entirely, but the controller
+    /// itself does not enforce that pairing — the headless config loader
+    /// does, before a <see cref="LiveSessionConnectOptions"/> is ever built.
+    /// </summary>
+    bool Probe = false);
 
 public interface IRuntimeLiveSessionFramePhase
 {

src/AcDream.Runtime/Session/LiveSessionController.cs

@@ -13,6 +13,14 @@ public enum LiveSessionStartStatus
     Connected,
     Deferred,
     Failed,
+    /// <summary>
+    /// Campaign LA slice LA2: a <see cref="LiveSessionConnectOptions.Probe"/>
+    /// session connected, received (and reported) the character roster, and
+    /// gracefully disconnected BEFORE selection/EnterWorld — deliberately a
+    /// SUCCESS variant of the <see cref="NoCharacters"/> early-exit shape
+    /// (same <c>StopCore</c> teardown), not a failure.
+    /// </summary>
+    ProbeComplete,
 }
 
 public readonly record struct LiveSessionOwnershipSnapshot(
@@ -647,6 +655,24 @@ public sealed class LiveSessionController
                     return new LiveSessionStartResult(LiveSessionStartStatus.Deferred);
             }
 
+            // Campaign LA slice LA2: the probe short-circuit lands here —
+            // right after the roster report, before TrySelectCharacter ever
+            // runs — so a probe session never reaches selection,
+            // ApplySelectedCharacter, or EnterWorld. This mirrors the
+            // NoCharacters early-exit immediately below (same StopCore
+            // teardown), the deliberate difference being the returned status
+            // is a SUCCESS, not a failure. Non-probe callers (options.Probe
+            // is false by default) fall straight through to the unchanged
+            // selection/enter path below — byte-identical to pre-LA2
+            // behavior.
+            if (options.Probe)
+            {
+                Console.WriteLine(
+                    "live: probe complete — disconnecting before EnterWorld");
+                StopCore();
+                return new LiveSessionStartResult(LiveSessionStartStatus.ProbeComplete);
+            }
+
             if (characters is null
                 || !TrySelectCharacter(
                     characters,

src/AcDream.Runtime/Session/LiveSessionHost.cs

@@ -316,6 +316,8 @@ public sealed class LiveSessionHost : IRuntimeSessionCommands
                 RuntimeSessionStartStatus.Deferred,
             LiveSessionStartStatus.Failed =>
                 RuntimeSessionStartStatus.Failed,
+            LiveSessionStartStatus.ProbeComplete =>
+                RuntimeSessionStartStatus.ProbeComplete,
             _ => throw new ArgumentOutOfRangeException(
                 nameof(result),
                 result.Status,