host: split src/host/main.cpp into translation units

main.cpp (4.5k lines) -> main.cpp (entry, args, synthetic benchmark) plus
vulkan_window, live_client, frame_policy, native_audio, texture_decode,
render_state, input_keymap, host_util. Bodies are moved verbatim; multi-line
VulkanWindow / NativeAudioEngine / FrameRatePolicy members are defined out of
line; one-line accessors stay inline. Anonymous namespace -> mt_host.

No behavior change: ctest 15/15, port_gate macos+android PASS, desktop
--live-client login screen and synthetic --draws runs unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/host/live_client.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#ifdef MT_NATIVE_HAS_LIVE_CLIENT
+#include <string>
+
+namespace mt_host {
+
+class VulkanWindow;
+
+// --py-exec: a test hook run once the auto-login run reaches the GameWindow (or, with --login-screen,
+// once the LoginWindow is up).
+extern std::string g_py_exec_after_login;
+// --perf-log / MT_PERF_LOG=1: src/host/perf_log.h.
+extern bool g_perf_log;
+
+// Boots the 40250 client (PythonBoot) from client_dir and runs it inside the renderer until it quits.
+int run_live_client(
+    VulkanWindow& renderer,
+    const std::string& client_dir,
+    int frames,
+    int fake_mobs,
+    bool gpu_skinning,
+    bool native_terrain,
+    bool login_screen,
+    bool selection_screen,
+    const std::string& live_server_spec,
+    const std::string& capture_out,
+    const std::string& screenshot_out);
+
+} // namespace mt_host
+#endif