Add roadmap task: validate Vulkan backend against a real game

Adds a fourth Current Task: exercise the Vulkan capture path end-to-end on
Sphere Spectacle (Steam appid 1123040) via both early-presence methods
(Auto-attach and the implicit coop_vk_layer), verifying capture, image
correctness, and no performance regression.

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

README.md

@@ -151,6 +151,18 @@ default** and covers anything the hooked path doesn't.
   (`IDirectInputDevice8::GetDeviceState/GetDeviceData`) don't see it. Add hooks for
   those paths to synthesize the forwarded input there too.
 
+- **Validate the Vulkan backend against a real game.** Exercise the Vulkan capture path
+  end-to-end on a shipping title — **Sphere Spectacle** (Steam appid 1123040,
+  `start steam://rungameid/1123040`) — not just `coop_mock_game`. Cover **both** early-presence
+  methods: **Auto-attach** (suspended-launch + inject + resume, so the hook arms before the game
+  calls `vkCreateInstance`) and the implicit **capture layer** (`coop_vk_layer` registered for the
+  game). For each, confirm: (a) it *captures* — frames reach the shared texture and advance; (b) the
+  mirrored image is *correct* — right resolution, letterboxed, correct colors/brightness (no
+  `*_SRGB` darkening, no BGRA/RGBA swizzle error), checked against a WGC reference and saved
+  screenshots; and (c) it doesn't *regress performance* — measure the game's present cadence /
+  frame time with capture off vs on and confirm no material drop (the per-frame `vkCmdCopyImageToBuffer`
+  read-back + present-semaphore re-chain must stay off the game's critical path).
+
 ## Building
 
 Requirements: Windows 10/11, Visual Studio 2022 (MSVC + C++ workload), CMake ≥ 3.21.