wip: Campaign LA LA1 fix round — INCOMPLETE, stopped mid-task

Agent was stopped for token budget partway through the LA1 review fix
round. Landed here: F1 best-effort SessionStatusWriter, F2 App reader
tolerance (paths/mode), F5 argument-parsing hardening, plus new tests.
NOT DONE: F4 shared-fixture production shape (was the next step), F3
reconnect disconnected edge + recorded limitation, F6 exited
idempotency/reasons, F7 structural redaction test, F8 platform-guard
test + comment fix, optional RuntimeOptions PrintMembers redaction.

Build/test state UNVERIFIED at this commit. Next session: finish the
remaining findings, run the suites, then narrow re-review.

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

src/AcDream.App/Configuration/SessionConfigurationLoader.cs

@@ -154,5 +154,31 @@ internal static class SessionConfigurationLoader
             throw new SessionConfigurationException(
                 $"Session '{session.Id}' statusFile must be a non-empty path when present.");
         }
+
+        ValidateMode(session);
+    }
+
+    /// <summary>
+    /// Campaign LA slice LA1 review fix (F2): <c>mode</c> is Headless-only
+    /// on the App host — the graphical host has no probe concept (LA2
+    /// builds the probe in Headless only). An absent field is today's ONLY
+    /// App behavior (play); <c>"probe"</c> gets a specific, actionable
+    /// message instead of a cryptic unmapped-member JSON error; anything
+    /// else is a plain configuration error.
+    /// </summary>
+    private static void ValidateMode(SessionDescriptor session)
+    {
+        if (session.Mode is null)
+            return;
+
+        if (string.Equals(session.Mode, "probe", StringComparison.Ordinal))
+        {
+            throw new SessionConfigurationException(
+                $"Session '{session.Id}' has mode 'probe'; probe sessions "
+                + "are headless-only and cannot run on the graphical host.");
+        }
+
+        throw new SessionConfigurationException(
+            $"Session '{session.Id}' has unsupported mode '{session.Mode}'.");
     }
 }