fix(launcher): Campaign LA LA3 review fixes — contract paths omission, probe composition, graceful stop, hygiene

Opus review of LA3 returned FIX FIRST; this addresses every finding in
scope (F1-F5, F7-F12; F6 CI-lane addition excluded per instructions):

- F1 (CRITICAL): SessionProcessSettings.Paths is now nullable and left
  null by SessionConfigComposer unless a caller supplies overrides, so
  the JSON key is entirely absent instead of "paths":{} — the App-side
  loader's strict UnmappedMemberHandling.Disallow would otherwise reject
  every gui/guiSelect session-config document at load.
- F2: added SessionConfigComposer.ComposeProbe and a nullable
  SessionDescriptor.Mode field ("probe", omitted for normal play) per
  the pinned contract — no character/policy/plugins/loginCommands.
- F3: LauncherProcessSupervisor.Stop now tries
  ILauncherChildProcess.TryRequestGracefulStop (Linux: libc SIGINT via
  LibraryImport, K4-proven graceful headless logout) before
  CloseMainWindow. Windows has no reliable no-window-console equivalent
  today; filed docs/ISSUES.md #397 with the CREATE_NEW_PROCESS_GROUP +
  CTRL_BREAK fix direction. Stop()'s blocking-timeout contract is now
  documented for LA4.
- F4: LauncherProfileStore.Save chmods the Linux temp file to 0600
  immediately after creation, before any credential is serialized;
  failure paths and Load() clean up a stale .tmp.
- F5: added LauncherCoreDependencyBoundaryTests asserting Launcher.Core
  references exactly AcDream.Platform and no packages.
- F7: StatusEventParser.Parse no longer throws on a whitespace/null
  line; StatusFileTailer.ReadNewEvents swallows the File.Exists/open
  TOCTOU window (FileNotFoundException/DirectoryNotFoundException/
  IOException) instead of throwing.
- F8: Start() now kills (entire process tree) and disposes a child that
  started successfully but failed while being fed its stdin password,
  instead of orphaning it.
- F9: SetState is monotonic — once Exited, no later transition applies
  or fires StateChanged, closing a Start()-path race where a
  synchronously-exiting child could be "resurrected" to Running.
- F10: CharacterIdFormat.TryParse now requires the "0x" prefix (an
  unprefixed hand-typed decimal id is also valid hex and was silently
  misread); a parsed id of 0 is treated as unusable and falls back to
  the name selector; LauncherProfileStore.MergeRoster normalizes both
  sides through TryParse/ToHexString instead of raw string equality, so
  a legacy unprefixed-hex row self-heals via name match instead of
  duplicating.
- F11: StatusCharacterEntry.SecondsGreyedOut is now uint, matching
  CharacterRosterEntry and the host writer.
- F12: added MalformedStatusEvent, returned for a recognized `e` whose
  payload doesn't match its shape, distinguished from UnknownStatusEvent
  (an unrecognized `e`).

AllowUnsafeBlocks was added to AcDream.Launcher.Core.csproj — required
by the LibraryImport source generator's function-pointer marshalling
stub for F3's Linux SIGINT P/Invoke.

Verification: dotnet build AcDream.slnx -c Release green (0 errors);
dotnet test tests/AcDream.Launcher.Core.Tests -c Release green at 94/94
on native Windows and under WSL (Ubuntu, verified across multiple runs
for the timing-sensitive SIGINT/sharing-violation tests, no flakes
observed).

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

src/AcDream.Launcher.Core/Launching/SessionConfigComposer.cs

@@ -52,13 +52,7 @@ public static class SessionConfigComposer
         ArgumentNullException.ThrowIfNull(paths);
         ArgumentException.ThrowIfNullOrWhiteSpace(sessionId);
 
-        string sessionDirectory = Path.Combine(
-            paths.CacheDirectory,
-            "launcher",
-            "sessions",
-            sessionId);
-        string configFilePath = Path.Combine(sessionDirectory, "session.json");
-        string statusFilePath = Path.Combine(sessionDirectory, "status.jsonl");
+        (string configFilePath, string statusFilePath) = BuildSessionPaths(paths, sessionId);
 
         SessionCharacterSelector? selector = character.LaunchMode == LaunchMode.GuiSelect
             ? null
@@ -92,7 +86,69 @@ public static class SessionConfigComposer
         {
             Process = new SessionProcessSettings
             {
-                Paths = new SessionPathOverrides(),
+                Content = new SessionContentDescriptor
+                {
+                    DatDirectory = install.DatDirectory,
+                    PreparedAssetPath = install.PreparedAssetPath,
+                },
+            },
+            Sessions = [descriptor],
+        };
+
+        return new ComposedSessionConfig(
+            sessionId,
+            configFilePath,
+            statusFilePath,
+            document);
+    }
+
+    /// <summary>
+    /// Builds a probe session-config document (Campaign LA plan §LA2/
+    /// §LA3 review finding F2): the session carries <c>mode: "probe"</c>,
+    /// no <c>character</c> selector, and no <c>policy</c> — the host
+    /// reports the account's character roster over the status stream and
+    /// exits without entering the world. <c>plugins</c>/<c>loginCommands</c>
+    /// don't apply to a probe and are always omitted, exactly like an
+    /// empty configured set on a normal session.
+    /// </summary>
+    public static ComposedSessionConfig ComposeProbe(
+        ServerProfile server,
+        AccountProfile account,
+        LauncherInstallRecord install,
+        ApplicationPathSet paths,
+        string sessionId)
+    {
+        ArgumentNullException.ThrowIfNull(server);
+        ArgumentNullException.ThrowIfNull(account);
+        ArgumentNullException.ThrowIfNull(install);
+        ArgumentNullException.ThrowIfNull(paths);
+        ArgumentException.ThrowIfNullOrWhiteSpace(sessionId);
+
+        (string configFilePath, string statusFilePath) = BuildSessionPaths(paths, sessionId);
+
+        var descriptor = new SessionDescriptor
+        {
+            Id = sessionId,
+            Mode = "probe",
+            Endpoint = new SessionEndpointDescriptor
+            {
+                Host = server.Host,
+                Port = server.Port,
+            },
+            Account = account.Account,
+            Character = null,
+            Policy = null,
+            Credential = new SessionCredentialDescriptor(),
+            Plugins = null,
+            LoginCommands = null,
+            LoginCommandDelayMs = null,
+            StatusFile = statusFilePath,
+        };
+
+        var document = new SessionConfigDocument
+        {
+            Process = new SessionProcessSettings
+            {
                 Content = new SessionContentDescriptor
                 {
                     DatDirectory = install.DatDirectory,
@@ -149,9 +205,28 @@ public static class SessionConfigComposer
     public static string Serialize(SessionConfigDocument document) =>
         JsonSerializer.Serialize(document, SerializerOptions);
 
+    private static (string ConfigFilePath, string StatusFilePath) BuildSessionPaths(
+        ApplicationPathSet paths,
+        string sessionId)
+    {
+        string sessionDirectory = Path.Combine(
+            paths.CacheDirectory,
+            "launcher",
+            "sessions",
+            sessionId);
+
+        return (
+            Path.Combine(sessionDirectory, "session.json"),
+            Path.Combine(sessionDirectory, "status.jsonl"));
+    }
+
     private static SessionCharacterSelector BuildSelector(CharacterProfile character)
     {
-        if (CharacterIdFormat.TryParse(character.Id, out uint id))
+        // A parsed id of 0 is not a usable selector — both host loaders
+        // (App/Headless) reject `id: 0` outright, so falling through to
+        // the name selector here is the only shape that reaches a real
+        // character (Campaign LA plan §LA3 review finding F10).
+        if (CharacterIdFormat.TryParse(character.Id, out uint id) && id != 0)
         {
             return new SessionCharacterSelector { Id = id };
         }