Fix hooked video darkening for *_SRGB-backbuffer games

The host sampled the shared backbuffer copy through a view typed exactly like the
game's backbuffer. For games whose backbuffer is an *_SRGB format (e.g. Life is
Strange: Before the Storm -- confirmed R8G8B8A8_UNORM_SRGB / fmt 29 via the hook
log), the GPU decoded sRGB->linear on the sample, and the host then wrote those
linear values straight to its plain-UNORM swapchain with no re-encode, so the
mirror came out noticeably darker than the game.

Sample the copy as the plain-UNORM sibling of the format (srgb_to_unorm) so the
bytes pass through unchanged -- matching what WGC already does. The UNORM and
*_SRGB formats share a typeless group, so CopyResource from the producer's sRGB
texture into the host's UNORM copy is allowed. Non-sRGB formats are unaffected.

Adds srgb_format_test locking the mapping. All 6 tests pass.

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

tests/CMakeLists.txt

@@ -59,6 +59,12 @@ target_link_libraries(audio_hook_test PRIVATE
 
 add_test(NAME audio_hook_test COMMAND audio_hook_test)
 
+# Unit test for the sRGB->UNORM format mapping the hooked video path uses to avoid
+# darkening *_SRGB-backbuffer games. Header-only, no device.
+add_executable(srgb_format_test srgb_format_test.cpp)
+target_include_directories(srgb_format_test PRIVATE ${CMAKE_SOURCE_DIR}/host/src)
+add_test(NAME srgb_format_test COMMAND srgb_format_test)
+
 # In-process self-test for the Present-hook video path. Reuses the shipping
 # present_hook.cpp and drives a real D3D11 swapchain in the same process, so it
 # exercises the IDXGISwapChain::Present inline hook, the shared keyed-mutex