M2(Vulkan layer): registry register/unregister + opt-in Injection-panel checkbox

Host helper register_vk_layer/unregister_vk_layer registers coop_vk_layer's
manifest as a per-user (HKCU, no admin) implicit Vulkan layer and writes the
scoping file (%TEMP%\coop_vk_target.txt) naming the target game's exe, so the
layer captures only that game and stays inert for every other Vulkan app. The
Injection panel gains an opt-in "Set up Vulkan layer" checkbox (next to Auto
re-attach, which the too-late banner points at); it registers on tick,
unregisters on untick, and the panel destructor unregisters on host exit so no
stale registration is left behind.

Verified live: a registry-registered layer is loaded by the loader for a fresh
vk launch (no env vars) and the scoping matches (active=1), then cleans up.

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

host/CMakeLists.txt

@@ -21,6 +21,7 @@ add_executable(coop_host WIN32
 	src/audio/audio_loopback.cpp
 	src/audio/audio_overrides.cpp
 	src/audio/process_loopback_capture.cpp
+	src/vk_layer_setup.cpp
 	src/test_harness.cpp)
 
 target_include_directories(coop_host PRIVATE src)