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/frame_policy.h

@@ -0,0 +1,50 @@
+#pragma once
+
+#ifdef MT_NATIVE_HAS_LIVE_CLIENT
+#include <string>
+
+namespace mt_host {
+
+// Command-line overrides, defined in frame_policy.cpp.
+extern int g_fps_cap;
+extern int g_refresh_rate;
+extern int g_fps_mode;
+extern int g_idle_fps;
+extern double g_idle_after_s;
+
+// The frame-rate setting (system option dialog, platform/EterBase/FrameRateMode.h) turned into a frame
+// rate, a display refresh rate and the ADPF budget; --fps-cap / --refresh-rate pin those for a test run.
+// Frames the player does not watch closely cost the same power as the ones they do, so after a while
+// without input (and no auto hunt running) the rate drops to --idle-fps until the next touch. The display
+// is asked for 60 Hz unless the setting wants more: a 120 Hz panel scanning out a 60 fps game costs power
+// for nothing.
+class FrameRatePolicy {
+public:
+    struct Decision {
+        int mode = -1;
+        int fps = 0;         // frames per second the loop aims at
+        int display_hz = 0;  // the refresh rate asked of the display
+        bool idle = false;
+        bool operator==(const Decision&) const = default;
+    };
+
+    bool pinned() const { return g_fps_cap > 0 || g_refresh_rate > 0; }
+
+    void load();
+
+    Decision decide(double idle_seconds, bool auto_hunt);
+
+    int max_hz() const { return max_hz_; }
+
+private:
+    static std::string settings_path();
+
+    void save(int mode);
+
+    std::string path_;
+    int saved_mode_ = -1;
+    int max_hz_ = 60;
+};
+
+} // namespace mt_host
+#endif