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>

.agents/skills/metin2-40250-parity-audit/references/project-map.md

@@ -33,7 +33,7 @@
 - `src/port/<Lib>/<File>.{h,cpp}` — mirror of each 40250 `logic`/`python` unit (see SKILL.md "Code layout").
 - `src/platform/` — implementations of the mirrored 40250 platform classes (D3D8 recorder, Granny,
   Miles, SpeedTree stand-in, window/input, ScriptLib boot).
-- `src/host/` — SDL3 + Vulkan host (`main.cpp`); `src/gr2/` — Granny reader; `src/codepage/` — code pages.
+- `src/host/` — SDL3 + Vulkan host (`main.cpp` entry, `live_client.cpp` loop, `vulkan_window.cpp` renderer); `src/gr2/` — Granny reader; `src/codepage/` — code pages.
 - `third_party/` — vendored cpython 2.7.18, cryptopp, freetype, minilzo.
 - `tests/port/` — port tests and the fake login server; `tests/gr2/` — Granny reader tests.
 - `android/` — SDLActivity shell, `android/build.sh`, `android/push-client.sh`.