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/srgb_format_test.cpp

@@ -0,0 +1,41 @@
+// Unit test for srgb_to_unorm (host/src/capture/dxgi_format.hpp): the hooked
+// video path samples the shared backbuffer copy as plain UNORM so an *_SRGB game
+// backbuffer isn't darkened by an sRGB->linear decode the host never re-encodes.
+// Locks the format mapping that fix depends on. No device needed.
+
+#include <cstdio>
+
+#include "capture/dxgi_format.hpp"
+
+using namespace coop;
+
+namespace
+{
+int g_failures = 0;
+void expect(DXGI_FORMAT in, DXGI_FORMAT want, const char* what)
+{
+	const DXGI_FORMAT got = srgb_to_unorm(in);
+	if (got != want)
+	{
+		std::printf("  FAIL: %s (got %d, want %d)\n", what, static_cast<int>(got), static_cast<int>(want));
+		++g_failures;
+	}
+}
+} // namespace
+
+int main()
+{
+	// sRGB formats map to their plain-UNORM sibling.
+	expect(DXGI_FORMAT_R8G8B8A8_UNORM_SRGB, DXGI_FORMAT_R8G8B8A8_UNORM, "R8G8B8A8 sRGB -> UNORM");
+	expect(DXGI_FORMAT_B8G8R8A8_UNORM_SRGB, DXGI_FORMAT_B8G8R8A8_UNORM, "B8G8R8A8 sRGB -> UNORM");
+	expect(DXGI_FORMAT_B8G8R8X8_UNORM_SRGB, DXGI_FORMAT_B8G8R8X8_UNORM, "B8G8R8X8 sRGB -> UNORM");
+
+	// Non-sRGB formats pass through unchanged.
+	expect(DXGI_FORMAT_R8G8B8A8_UNORM, DXGI_FORMAT_R8G8B8A8_UNORM, "R8G8B8A8 UNORM passthrough");
+	expect(DXGI_FORMAT_B8G8R8A8_UNORM, DXGI_FORMAT_B8G8R8A8_UNORM, "B8G8R8A8 UNORM passthrough");
+	expect(DXGI_FORMAT_R10G10B10A2_UNORM, DXGI_FORMAT_R10G10B10A2_UNORM, "R10G10B10A2 passthrough");
+	expect(DXGI_FORMAT_R16G16B16A16_FLOAT, DXGI_FORMAT_R16G16B16A16_FLOAT, "RGBA16F passthrough");
+
+	std::printf(g_failures == 0 ? "SRGB FORMAT TEST PASS\n" : "SRGB FORMAT TEST FAILED (%d)\n", g_failures);
+	return g_failures == 0 ? 0 : 1;
+}