Persist ImGui panel layout to disk; quiet audio set_audio_ring log spam

Layout persistence: re-enable io.IniFilename (was nullptr "for the spike"),
anchored to a coop_layout.ini next to the exe so window positions/sizes survive
restarts even when Steam launches us under the donor appid (CWD is unreliable).
Path is UTF-8 for ImGui's file IO. When a saved layout is restored at startup,
suppress the computed-default force so it does not clobber the user's positions;
Reset layout (and a fresh install with no .ini) still applies the default.

Log spam: the worker thread re-attaches every audio ring every tick (idempotent),
and set_audio_ring logged unconditionally, flooding the log. Only log when the
ring pointer actually changes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/imgui_layer.hpp

@@ -1,6 +1,8 @@
 // Owns ImGui setup/teardown for the Win32 + DX11 backends.
 #pragma once
 
+#include <string>
+
 #include <d3d11.h>
 #include <windows.h>
 
@@ -22,6 +24,9 @@ public:
 
 private:
 	bool initialized_ = false;
+	// Backing storage for io.IniFilename (ImGui keeps the pointer, not a copy), so the
+	// layout .ini path must outlive the context. Empty until init() sets it.
+	std::string ini_path_;
 };
 
 } // namespace coop