fix(host): break InputBar/CommandHelpWindow/MainWindow DI cycle

J + J2 closed a singleton cycle:
  InputBar.ctor -> CommandHelpWindow (J2)
  CommandHelpWindow.ctor -> MainWindow (J)
  MainWindow.ctor -> InputBar (pre-existing)

MS.DI does not detect cycles through FactoryCallSite registrations,
so resolution recursed silently on the async plugin-init thread until
the worker died with an uncatchable StackOverflowException. Dalamud's
LoadAsync task never resolved; the plugin UI hung on "Enabling..."
with no exception in the log. First triggered at Plugin.cs:289
(TypingIpc.ctor needs InputBar).

Fix: break the cycle on the laziest edge.
  - CommandHelpWindow.ctor no longer takes MainWindow.
  - New AttachMainWindow setter wired in
    CommandHelpWindowInitHostedService.StartAsync, mirroring the
    existing §6.2 MessageList.AttachPayloadHandler pattern.
  - UpdateContent throws InvalidOperationException if the setter
    never ran, so a future regression fails loudly instead of a
    silent NullRef during input draw.

Also enable UseDefaultServiceProvider(ValidateOnBuild + ValidateScopes)
so future ConstructorCallSite cycles throw at Build time instead of
silently hanging. Catches reflection-based registrations; will not
catch FactoryCallSite cycles like this one (those still need code review).

Verified via 6 enable/disable cycles in-game; plugin loads cleanly,
Hosting starts, FilterAllTabs completes, command help popup renders
for /em and /say (exercises AttachMainWindow), hover counter ticks
(exercises PayloadHandlerInitHostedService AddonLifecycle wiring).

HellionChat/Infrastructure/Hosting/InitHostedServices.cs

@@ -3,7 +3,9 @@ using Dalamud.Plugin;
 using HellionChat.Integrations;
 using HellionChat.Ipc;
 using HellionChat.Themes;
+using HellionChat.Ui;
 using HellionChat.Ui.Components;
+using HellionChat.Ui.Windows;
 using Microsoft.Extensions.Hosting;
 
 namespace HellionChat.Infrastructure.Hosting;
@@ -142,3 +144,23 @@ internal sealed class PayloadHandlerInitHostedService(
         });
     }
 }
+
+// Wires MainWindow into CommandHelpWindow post-container-build. CommandHelpWindow
+// cannot take MainWindow as a ctor-param because that would close the cycle
+// InputBar -> CommandHelpWindow -> MainWindow -> InputBar (MS.DI does not catch
+// it through FactoryCallSite registrations and the resolve recurses silently).
+// Both singletons exist by host.StartAsync time, so this is the first safe point
+// to wire the setter — same §6.2 pattern as MessageList.AttachPayloadHandler.
+internal sealed class CommandHelpWindowInitHostedService(
+    CommandHelpWindow commandHelpWindow,
+    MainWindow mainWindow
+) : IHostedService
+{
+    public Task StartAsync(CancellationToken cancellationToken)
+    {
+        commandHelpWindow.AttachMainWindow(mainWindow);
+        return Task.CompletedTask;
+    }
+
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}

HellionChat/PluginHostFactory.cs

@@ -29,6 +29,15 @@ internal static class PluginHostFactory
                 logging.AddDalamudLogging(dependencies.PluginLog);
                 logging.SetMinimumLevel(LogLevel.Trace);
             })
+            // ValidateOnBuild eagerly instantiates every singleton at Build time
+            // so missing registrations / ConstructorCallSite cycles throw on
+            // load instead of producing a silent hang. ValidateScopes is cheap
+            // (we only use singletons) but guards against future Scoped misuse.
+            .UseDefaultServiceProvider(o =>
+            {
+                o.ValidateOnBuild = true;
+                o.ValidateScopes = true;
+            })
             .ConfigureServices(services => ConfigureServices(services, plugin, dependencies))
             .Build();
     }
@@ -268,9 +277,11 @@ internal static class PluginHostFactory
             sp.GetRequiredService<Ui.Components.InputBar>(),
             sp.GetRequiredService<ILogger<InputPreview>>()
         ));
+        // No MainWindow ctor-param: breaks the InputBar -> CommandHelpWindow ->
+        // MainWindow -> InputBar singleton cycle. MainWindow is wired post-build
+        // via CommandHelpWindowInitHostedService.
         services.AddSingleton(sp => new CommandHelpWindow(
             sp.GetRequiredService<Ui.Components.ChunkRenderer>(),
-            sp.GetRequiredService<Ui.Windows.MainWindow>(),
             sp.GetRequiredService<ILogger<CommandHelpWindow>>()
         ));
         services.AddSingleton(sp => new SeStringDebugger(sp.GetRequiredService<Plugin>()));
@@ -312,6 +323,10 @@ internal static class PluginHostFactory
             sp.GetRequiredService<PayloadHandler>(),
             sp.GetRequiredService<Ui.Components.MessageList>()
         ));
+        services.AddHostedService(sp => new CommandHelpWindowInitHostedService(
+            sp.GetRequiredService<Ui.CommandHelpWindow>(),
+            sp.GetRequiredService<Ui.Windows.MainWindow>()
+        ));
     }
 
     private static PayloadHandler MakePayloadHandler(IServiceProvider sp) =>

HellionChat/Ui/CommandHelpWindow.cs

@@ -13,20 +13,21 @@ namespace HellionChat.Ui;
 internal sealed class CommandHelpWindow : Window
 {
     private readonly ChunkRenderer _chunkRenderer;
-    private readonly Windows.MainWindow _mainWindow;
     private readonly ILogger<CommandHelpWindow> _logger;
 
+    // Setter-injected post-ctor to break the InputBar -> CommandHelpWindow ->
+    // MainWindow -> InputBar singleton cycle (MS.DI does not detect cycles
+    // through FactoryCallSite registrations). Wired in
+    // CommandHelpWindowInitHostedService.StartAsync, same §6.2 pattern as
+    // MessageList.AttachPayloadHandler.
+    private Windows.MainWindow? _mainWindow;
+
     private ReadOnlySeString? _commandDescription;
 
-    internal CommandHelpWindow(
-        ChunkRenderer chunkRenderer,
-        Windows.MainWindow mainWindow,
-        ILogger<CommandHelpWindow> logger
-    )
+    internal CommandHelpWindow(ChunkRenderer chunkRenderer, ILogger<CommandHelpWindow> logger)
         : base("command help##chat2-commandhelp")
     {
         _chunkRenderer = chunkRenderer;
-        _mainWindow = mainWindow;
         _logger = logger;
 
         Flags =
@@ -44,8 +45,18 @@ internal sealed class CommandHelpWindow : Window
         _ = _logger;
     }
 
+    internal void AttachMainWindow(Windows.MainWindow mainWindow) => _mainWindow = mainWindow;
+
     public void UpdateContent(ReadOnlySeString commandDesc)
     {
+        // Loud-fail if the HostedService didn't run AttachMainWindow before
+        // the first slash-command call — better than a silent NullRef during
+        // input draw.
+        if (_mainWindow is null)
+            throw new InvalidOperationException(
+                "CommandHelpWindow.UpdateContent called before AttachMainWindow."
+            );
+
         _commandDescription = commandDesc;
 
         var width = 350;