Render through public Qt API: OpenGL and Vulkan backends, --graphics-api

Replace the QRhi rendering in OverlayController (private Qt API) with the
render backends of the vrdashboard template (src/render/), which use public
Qt API only (QQuickRenderTarget::fromOpenGLTexture / fromVulkanImage). The
binary then runs on another Qt 6 patch release, e.g. the Steam Frame's system
Qt.

The Vulkan backend also does what SteamVR requires and the QRhi path lacked:
the required instance/device extensions and the HMD's GPU (via
openvr_init::renderInterop()), VK_FORMAT_R8G8B8A8_UNORM, and the transition to
TRANSFER_SRC_OPTIMAL after each frame.

- New option --graphics-api opengl|vulkan (default opengl, no fallback).
- Shutdown() calls VR_Shutdown before the render target and Qt's device go
  (crash on the Steam Frame with Vulkan otherwise); it runs only once.
- Mouse y is flipped on every platform: SteamVR reports it in GL convention.
- Render delay 5 -> 16 ms (about 60 fps).
- GuiPrivate is no longer needed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/steamframe/PROGRESS.md

@@ -57,3 +57,48 @@
   `false`) is on. That's a chaperone-profile feature, so it belongs to R5 (iteration 2).
 - clang-format reflows comment blocks next to new comments. Separate new comments from
   existing commented-out code with a blank line, otherwise the diff grows.
+
+### Step 3: rendering through public API, `--graphics-api`, mouse y, shutdown order (2026-10-07)
+
+**Changes:**
+- **`src/render/`** (new): `RenderBackend`, `OpenGLRenderBackend`, `VulkanRenderBackend` from
+  the template (`4a5b090`).
+  - Changes to the template code: `VRDASH_*` → `OVRAS_*` names, origin note in the file
+    header, references to `OverlayController`.
+  - Formatted completely in OVRAS style.
+  - One comment in `VulkanRenderBackend.h` was wrapped by hand, because the automatic
+    wrapping had broken its column layout.
+- **`OverlayController`:**
+  - QRhi block replaced by `configureWindow` → `renderControl.initialize` →
+    `attachWindow` → `createRenderTarget`.
+  - `renderOverlay()` wraps the frame in `beginFrame`/`finishFrame`. It submits
+    `vrTextureBounds` + `vrTexture` while the context is current.
+  - `SetOverlayFromQRhiTexture`, the QRhi members and `QVulkanInstance` are removed.
+  - Render delay 5 → 16 ms (about 60 fps).
+- **Shutdown:** `Shutdown()` runs only once. It calls `VR_Shutdown()`, then frees the
+  backend's render target (`releaseRenderTarget`, `detachWindow`).
+- **Mouse y:** always flipped; the Linux un-flip and its TODO are removed.
+- **`openvr_init::renderInterop()`:** Vulkan extensions and HMD GPU from SteamVR.
+- **`main.cpp`:** parses `--graphics-api`, then `createRenderBackend` → `initialize` →
+  `QQuickWindow::setGraphicsApi` → `OverlayController`.
+- **`setup.*`:** `--graphics-api opengl|vulkan` (default `opengl`). Unknown or not built in
+  → error, exit code `GENERAL_FAILURE`.
+- **CMake:**
+  - `GuiPrivate` is gone.
+  - The Vulkan backend is built when `QT_FEATURE_vulkan` is set (`OVRAS_HAS_VULKAN`).
+
+**Results (x86_64, Qt 6.11.1):**
+- The build succeeds. The Vulkan backend is built in.
+- `objdump -T`: only `QVulkanDeviceFunctions::*` / `QVulkanFunctions::*` remain as
+  `Qt_6_PRIVATE_API`. Acceptance criterion R1 (symbols) is met.
+- Warnings 1463 → 1630. The new code itself has none; the additional ones come from
+  `openvr.h` in the three new translation units.
+- **Not tested yet:**
+  - running both backends, Vulkan validation, the snapshot test (step 5, with the
+    containers)
+  - mouse direction (Frame)
+
+**Risk:**
+- `MyTextField` opens the SteamVR keyboard on `activeFocus`. The template had to send the
+  offscreen window an explicit `FocusIn`, otherwise `activeFocusItem` stayed null. The `qt6`
+  branch doesn't send it. If text fields don't respond on the Frame, that is the cause (R4).