M2(Vulkan): implicit capture layer (coop_vk_layer) + chain dispatch + env test

A real chain-aware Vulkan implicit layer the loader inserts at vkCreateInstance
-- the reliable early-presence path for games that init Vulkan immediately,
which the inline-hook vk_hook can't catch. It intercepts vkCreateInstance /
Device / CreateSwapchainKHR / QueuePresentKHR via proper layer-chain dispatch
and does the same read-back capture (vkCmdCopyImageToBuffer -> swizzle ->
hook-owned D3D11 shared texture, with present-semaphore re-chaining) as vk_hook.

The loader/layer link structs (VkLayer*CreateInfo, VkNegotiateLayerInterface)
aren't in Vulkan-Headers, so they're hand-declared to interface version 2. Key
gotcha found via tracing: the loader tags those link structs with small internal
sType values (LOADER_INSTANCE_CREATE_INFO=47, _DEVICE=48), not the 1000000000
range -- matching the wrong value made the device-chain walk fail.

Scoping: an implicit layer loads into every Vulkan app, so it only *captures*
when COOP_VK_LAYER_FORCE is set (tests) or this process's image matches
%TEMP%\coop_vk_target.txt (the host writes it); otherwise pure pass-through.
mock_game_test registers it via VK_LAYER_PATH/VK_INSTANCE_LAYERS and decodes
frames through it. Ships at the bin root with its JSON manifest.

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

vk_layer/coop_vk_layer.json

@@ -0,0 +1,14 @@
+{
+    "file_format_version": "1.2.0",
+    "layer": {
+        "name": "VK_LAYER_coop_capture",
+        "type": "GLOBAL",
+        "library_path": ".\\coop_vk_layer.dll",
+        "api_version": "1.1.0",
+        "implementation_version": "1",
+        "description": "CoopAllTheThings Vulkan capture layer (mirrors the swap-chain image into a shared texture; captures only the host's target process)",
+        "disable_environment": {
+            "COOP_VK_LAYER_DISABLE": "1"
+        }
+    }
+}