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>

audit/port-map/EterLib/GrpImageTexture.cpp.json

@@ -29,8 +29,8 @@
       "status": "ADAPTED",
       "impl": [
         "src/host/dxt.cpp:decode",
-        "src/host/main.cpp:get_gpu_texture",
-        "src/host/main.cpp:upload_texture"
+        "src/host/vulkan_window.cpp:get_gpu_texture",
+        "src/host/vulkan_window.cpp:upload_texture"
       ],
       "test": "tests/port/dxt_decode_test.cpp",
       "note": "The platform CreateDDSTexture member is a stub; its level logic lives in the texture decode/upload the renderer does. mipmapCount = max(1, min(dwMipMapCount, MAX_MIPLEVELS=12)), level i read at sum(dwLinearSize>>2k) with CDXTCImage::Copy's dwLinearSize>>2i bytes (the rest zero, uninitialised in 40250), decoded to RGBA8 (no GPU DXT) and uploaded as file levels, not regenerated. Divergence: without DDSD_MIPMAPCOUNT, or with DDSD_PITCH, 40250 creates the extra levels uninitialised; here that is one level. IsLowTextureMemory's uTexBias level skip is not applied (the port's ms_isLowTextureMemory is false). The far-terrain look NEEDS_LIVE."
@@ -39,7 +39,7 @@
       "status": "ADAPTED",
       "impl": [
         "src/platform/EterLib/GrpImageTexture.cpp:CGraphicImageTexture::CreateFromMemoryFile",
-        "src/host/main.cpp:get_gpu_texture"
+        "src/host/vulkan_window.cpp:get_gpu_texture"
       ],
       "test": "tests/port/dxt_decode_test.cpp",
       "note": "The platform body records only the size and file name; the renderer decodes the bytes. 40250 dispatch is kept: a DXT DDS takes the CreateDDSTexture levels, anything else takes D3DXCreateTextureFromFileInMemoryEx(MipLevels=D3DX_DEFAULT), i.e. a full generated chain (vkCmdBlitImage linear, instead of D3DX's box filter). The invented is_ui_tex rule (no mips for icon/, ui/, locale/ and textures <=128px) was removed. Memory textures (mem:, 40250 CreateTexture levels=1) get one level. GRAPHICS_CAPS_HALF_SIZE_IMAGE/low-memory A4R4G4B4 re-encode is not applied."