docs: Campaign LA — pinned launch-contract schema COMMITTED into plan LA1

The LA3 Opus review process note was right: the contract both sides
implement lived only in orchestrator prompts, which is exactly the drift
mode the pin exists to prevent (and it produced the paths-key CRITICAL).
The schema, field rules, probe-mode discriminator, and status vocabulary
are now a binding plan section; amendments change this text first,
implementations second. Ledger: LA3 fix round dispatched.

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

src/AcDream.App/Composition/SessionStartComposition.cs

@@ -1,9 +1,14 @@
 using AcDream.Runtime;
+using AcDream.Runtime.Session;
 
 namespace AcDream.App.Composition;
 
 internal sealed record SessionStartDependencies(
-    Action<string> Log);
+    Action<string> Log,
+    /// <summary>Campaign LA slice LA1: no-op when no statusFile was
+    /// configured.</summary>
+    SessionStatusWriter StatusWriter,
+    string SessionId);
 
 /// <summary>
 /// Terminal startup phase. Every callback, command target, and frame root is
@@ -21,6 +26,9 @@ internal sealed class SessionStartCompositionPhase
     public void Start(FrameRootResult frame)
     {
         ArgumentNullException.ThrowIfNull(frame);
+        // Campaign LA slice LA1: "started" = session host start — the
+        // earliest point the graphical host actually attempts to connect.
+        _dependencies.StatusWriter.Started(_dependencies.SessionId);
         RuntimeSessionStartResult result =
             frame.GameRuntime.Session.Start(frame.GameRuntime.Generation);
         Report(result, _dependencies.Log);