Docs: Vulkan hooked capture in architecture + lessons; scope the opt-in layer

Architecture now lists Vulkan as a hooked producer (GPA interception + read-back,
early-presence required). Adds a lessons-learned bullet (can't late-hook Vulkan;
the present-semaphore re-chaining trap; the too-late heuristic; early-load
testing). mock_game_test docs note the suspended-launch capture + too-late check.
The Vulkan milestone is trimmed to its one remaining piece -- the opt-in implicit
layer for immediate-init games -- with the capture/banner/best-effort marked done.

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

README.md

@@ -44,9 +44,15 @@ read-backs the backbuffer with `GetRenderTargetData` (a D3D9 surface isn't D3D11
 swizzling BGRA→RGBA and uploading via a hook-owned D3D11 device — one path covers both plain
 D3D9 and D3D9Ex; **OpenGL** hooks `SwapBuffers` / `wglSwapBuffers` and reads the
 backbuffer with `glReadPixels` (for games that never touch DXGI, e.g. Phantom
-Brave). The host samples the copy as plain UNORM (`srgb_to_unorm`) so
-`*_SRGB`-backbuffer games mirror at correct brightness. **WGC remains the default**
-and covers anything the hooked path doesn't (Vulkan — see Roadmap).
+Brave); and **Vulkan** inline-hooks the `vulkan-1.dll` `vkGetInstanceProcAddr` export to
+intercept the resolution chain (`vkCreateInstance` / `vkCreateDevice` /
+`vkCreateSwapchainKHR` / `vkQueuePresentKHR`) and reads the presented image back with
+`vkCmdCopyImageToBuffer` — but only when the hook is present **before** the game initializes
+Vulkan (it caches its present pointer at init), so the Vulkan path needs early presence via
+Auto-attach (and an opt-in implicit layer — see Roadmap); a too-late attach shows a red
+relaunch banner and falls back to WGC. The host samples the copy as plain UNORM
+(`srgb_to_unorm`) so `*_SRGB`-backbuffer games mirror at correct brightness. **WGC remains the
+default** and covers anything the hooked path doesn't.
 
 ## Limitations
 
@@ -147,38 +153,24 @@ Conventions for every milestone below:
     nothing clipped/overlapping, colours/labels correct. A green unit test is not a substitute
     for looking at the product; this real-screenshot check is required, not optional.
 
-- **M2 — Vulkan capture.** The Vulkan **mock backend is done** (`render_vk.cpp`, selectable `vk`):
-  volk + Vulkan-Headers submodules, a real instance / device / swap chain, and a per-frame
-  `vkCmdClearColorImage` of the frame-counter colour (no pipeline/shaders → **no SPIR-V
-  toolchain**); it loads Vulkan via **volk** on purpose (the loader-bypass case the capture hook
-  must handle). `mock_game_test` runs a **liveness** check on it — its present pointer is cached
-  at init, so late injection can't hook it, which is exactly why the capture below is the hard
-  part. The remaining **capture hook** (Vulkan games mirror via **WGC** meanwhile — the default):
-  1. **Present interception (needs early presence).** Hook `vkGetInstanceProcAddr` /
-     `vkGetDeviceProcAddr` (the `vulkan-1.dll` exports) so a volk-using app resolves *our*
-     wrappers, and intercept `vkCreateDevice` / `vkCreateSwapchainKHR` to capture the
-     device/queue/images/format. Read the presented image back with `vkCmdCopyImageToBuffer` into
-     a host-visible buffer (the same read-back pattern as D3D10/D3D9/OpenGL — simpler and more
-     robust than a `VK_KHR_external_memory_win32` + keyed-mutex blit), swizzle if BGRA, and upload
-     via the hook-owned D3D11 device. **Testing needs the early-load path** (the mock loads the
-     hook before `vkCreateInstance`, e.g. via an env-var or the layer); the normal late-inject
-     `mock_game_test` flow can't catch a Vulkan present.
-  2. **Attach via auto-attach — best-effort by default, opt-in layer for reliability.** By
-     default, arming Auto-attach for a detected Vulkan game injects as early as possible on
-     relaunch (the existing poll-and-inject) — enough for games that initialize Vulkan a little
-     into startup. For games that init Vulkan immediately, an **opt-in** Injection-panel checkbox
-     ("Set up Vulkan layer") registers a per-user (HKCU, no admin) implicit Vulkan layer (thin
-     `coop_vk_layer.dll` + JSON manifest) scoped to that game's image name; on relaunch the loader
-     loads it at `vkCreateInstance` — guaranteed before init — and it wires up present capture +
-     IPC. The layer is removed when the option is unticked or the host exits, and self-deactivates
-     for any non-target app, so a stale registration (e.g. after a host crash) is harmless.
-  3. **Too-late detection + red banner.** A late-injected hook that finds `vulkan-1.dll` loaded
-     but no working hooked-present reports a *"Vulkan, injected too late"* status; the host
-     overlays a **red** banner on the mirror window: "Detected a Vulkan game — the hook attached
-     too late to mirror it with low latency. Enable Auto-attach (and, if it persists, 'Set up
-     Vulkan layer') and relaunch the game." **WGC keeps mirroring meanwhile** so the session stays
-     usable; the banner clears once the hooked path goes live. This is new end-user UI, so it must
-     keep M1's fit test (and the visual inspection) green.
+- **M2 — Vulkan: opt-in reliability layer (the one piece left).** The Vulkan **mock backend and
+  hooked capture are done and tested**: the hook inline-hooks the `vulkan-1.dll`
+  `vkGetInstanceProcAddr` export and intercepts the resolution chain (`vkCreateInstance` /
+  `vkCreateDevice` / `vkCreateSwapchainKHR` / `vkQueuePresentKHR`), reads the presented image back
+  with `vkCmdCopyImageToBuffer`, and re-chains the present's wait semaphores so the copy orders
+  after rendering. `mock_game_test` decodes Vulkan frames through it (via a suspended-launch +
+  early-load path) and verifies the **too-late detection**; the host shows the **red relaunch
+  banner** (verified live). **Best-effort auto-attach** already works (the existing poll-and-inject
+  re-injects on relaunch, the vk hook retries until `vulkan-1.dll` loads), enough for games that
+  don't initialize Vulkan instantly. Remaining:
+  - **Opt-in implicit Vulkan layer** for games that init Vulkan *immediately* (where even a
+    relaunch+auto-attach injects too late). A real chain-aware Vulkan layer (`coop_vk_layer.dll`
+    + JSON manifest) the loader loads at `vkCreateInstance` — guaranteed before init — registered
+    per-user (HKCU, no admin) and scoped to the target image, with an **opt-in** "Set up Vulkan
+    layer" Injection-panel checkbox that registers/unregisters it (self-deactivating for non-target
+    apps so a stale entry after a host crash is harmless). This is a separate component from the
+    inline-hook capture (layer-chain dispatch + manifest/registry + the checkbox + IPC handshake);
+    until it lands, the banner directs immediate-init Vulkan games to WGC.
 
   Each of 1–3 is its own commit; test the early/layer path against the mock (decode frames) and
   the too-late path (assert the status + banner fire).
@@ -292,17 +284,20 @@ ctest --test-dir build -C Debug --output-on-failure
   endpoint format). Skips cleanly if the machine has no audio endpoint.
 - **`mock_game_test`** — comprehensive capture/audio/hook stress test against
   **`coop_mock_game`** (an animated, frame-numbered A/V test game under
-  [`tools/mock_game`](tools/mock_game) with selectable **DX9 / DX9Ex / DX10 / DX11 / DX12 / OpenGL** backends and a
-  configurable WASAPI tone). It launches the game, injects `coop_hook.dll`, opens the
-  hook's shared video texture, and **decodes the frame number out of the captured pixels**
-  to assert the mirror sees a *monotonic, advancing* sequence for each backend (the bar
-  for no dropped / stale / out-of-order frames — what the DX12 rotating-backbuffer bug
-  broke). It launches the game at several **audio formats** (44100/48000/96000, PCM +
-  float) and asserts the hook measures each one's rate through the full inject path, then
-  injects with audio + video, checks both stream, cycles the audio subsystem off/on
-  (hook/unhook stress), and confirms the game never crashes and capture resumes. This
-  suite drove out five real audio races (see Lessons learned). Skips cleanly without a
-  D3D11 device.
+  [`tools/mock_game`](tools/mock_game) with selectable **DX9 / DX9Ex / DX10 / DX11 / DX12 /
+  OpenGL / Vulkan** backends and a configurable WASAPI tone). It launches the game, injects
+  `coop_hook.dll`, opens the hook's shared video texture, and **decodes the frame number out of
+  the captured pixels** to assert the mirror sees a *monotonic, advancing* sequence for each
+  backend (the bar for no dropped / stale / out-of-order frames — what the DX12 rotating-backbuffer
+  bug broke). **Vulkan** is special: since its present pointer is cached at init, the test launches
+  the mock **suspended**, injects, then resumes (the mock loads Vulkan and waits so the hook arms
+  first) to decode frames; it also late-injects a Vulkan game and asserts the **too-late** flag
+  trips (which drives the host's relaunch banner). It launches the game at several **audio formats**
+  (44100/48000/96000, PCM + float) and asserts the hook measures each one's rate through the full
+  inject path, then injects with audio + video, checks both stream, cycles the audio subsystem
+  off/on (hook/unhook stress), and confirms the game never crashes and capture resumes. This suite
+  drove out five real audio races (see Lessons learned). Skips cleanly without a D3D11 / Vulkan
+  device.
 
 ### Debugging the hooks against a real game
 
@@ -460,6 +455,20 @@ Non-obvious things that cost time and constrain the design:
   the frame-counter block at the GL *top* (`y = h - block`) to land at the captured image's
   top-left. The GL `SwapBuffers` hook now also bumps the shared present counter (it's the GL
   present), so the Video panel's present rate works for GL games too.
+- **Vulkan can't be late-hooked, and capturing its present needs semaphore surgery.** Vulkan
+  games cache `vkQueuePresentKHR` at init (often via volk, which bypasses the loader trampoline),
+  so late injection misses it — the hook must be in **before `vkCreateInstance`**. Catch the
+  resolution chain instead: inline-hook the `vulkan-1.dll` `vkGetInstanceProcAddr` export and hand
+  back wrappers for `vkCreateInstance` / `vkCreateDevice` / `vkCreateSwapchainKHR` /
+  `vkQueuePresentKHR` (so a volk app resolves *ours*), tracking the device / queue / images /
+  format; read the presented image back with `vkCmdCopyImageToBuffer` and upload it to the shared
+  texture on a hook-owned D3D11 device. The trap: the read-back submit must **wait on the present's
+  wait semaphores** (consume them) and **signal a fresh semaphore the real present then waits on**
+  — both your copy and the present can't wait the same binary semaphore. "Injected too late" is
+  detected as *vulkan-1.dll loaded + hook in for >4 s + we never saw `vkCreateDevice`* (the app set
+  everything up before us) → red relaunch banner, WGC meanwhile. Testing needs an **early-load**
+  path (suspended launch + inject + resume; the mock loads Vulkan and waits), since the normal
+  late-inject flow can't catch a Vulkan present.
 - **A render client that predates our injection has no knowable format — measure it.**
   We inject into already-running games, so we usually never see the game's
   `IAudioClient::Initialize`; the render-hook then assumes the device mix format for that