feat(util): add IPluginLogProxy interface and production wrapper

F12.2 closes the gap that F12.1 left open: MessageStore's ctor calls
Plugin.Log.Information inside Migrate0, which prevents an isolated xUnit
construction test (Dalamud.dll cannot load in the test AppDomain).

The proxy mirrors IPluginLog's full surface (Verbose/Debug/Information/
Info/Warning/Error/Fatal — both Info and Information as Dalamud exposes
them) with both single-string and Exception+string overloads, so the
~91 existing Plugin.Log.* call-sites become a drop-in rewrite to
Plugin.LogProxy.*.

A later DI-container adoption cycle (v1.5.x) may swap this for
Microsoft.Extensions.Logging's ILogger<T>; this commit is the
intermediate decorator step.

HellionChat/Plugin.cs

@@ -117,6 +117,11 @@ public sealed class Plugin : IAsyncDalamudPlugin
     // any service allocated in LoadAsync can read Plugin.PlatformUtil.
     internal static IPlatformUtil PlatformUtil { get; private set; } = null!;
 
+    // Log indirection over Dalamud's IPluginLog. Same rationale as PlatformUtil:
+    // call-sites read through LogProxy so MessageStore can be tested in
+    // isolation. Wired immediately after Dalamud injects Log.
+    internal static IPluginLogProxy LogProxy { get; private set; } = null!;
+
     // Idempotency guard — Dalamud may fire DisposeAsync twice in a reload race.
     private int _disposeStarted;
 
@@ -162,6 +167,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
         // needs Util.* — services then read Plugin.PlatformUtil instead of
         // hitting the Dalamud static surface directly.
         PlatformUtil = new DalamudPlatformUtil();
+        LogProxy = new DalamudPluginLogProxy(Log);
 
         // Schema gate: v1.4.x requires config v16. Users on older schemas
         // must install v1.4.2 first to run the migration chain.