feat(render): draw the retained UI on Vulkan, and fix the pass it exposed

Campaign V slice V6d, commit 3 of 3 — the evidence commit, which turned out to also be a bug-fix commit.

VulkanBringUpHost now builds a real UiHost and DebugLineRenderer on the Vulkan device and draws them after the V6c verification scene, in their own single-sampled load/store passes against the backbuffer — the same shape the GL client's HUD phase has. Nothing in the retained stack is backend-aware: UiRoot walks a real widget tree, each widget draws through UiRenderContext, and UiHost.Draw brackets it with TextRenderer.Begin/Flush. What it cannot be is the game's own UI, because the retail tree is built from LayoutDesc and DAT chrome by TextureCache, which stays a GL type until V4t; the sprites here are generated instead. The widget rectangles are authored at known pixel offsets from the top-left and nothing is mirror-symmetric, so a wrong Y flip would put the title bar at the bottom.

The frame this produced was wrong, and usefully so. Whole runs of the debug-line figure were missing. Vulkan's rasterization-order guarantees are scoped to one render-pass instance; between two instances writing the same attachment there is no implicit ordering, and that includes a multisample RESOLVE, which is part of the render pass and therefore equally unordered against what follows. TransitionBackbufferForRendering emitted its acquire barrier once per frame and returned for every pass after the first, so the second and third passes raced the first one's resolve. V6c's frame had exactly one backbuffer pass and could not see this; V6d's has three. A later backbuffer pass now gets a colour-attachment dependency instead of nothing, and keeps ColorAttachmentOptimal as its old layout rather than Undefined, which would have licensed discarding everything drawn so far. Every line renders continuously afterwards.

Inspection of artifacts/vk-ui/vulkan-bringup.png against the authored layout, by pixel probe:

The header panel is authored at (24,18), 420x96. Its tiled chrome fills exactly x 24..443 and y 18..113 — one pixel outside on any edge is the clear colour. The tile's lit edge appears at the top and left of every cell, so texture row 0 lands at the top and the V axis is not flipped. Both labels read left to right, right side up, through the font-coverage branch. The nested panel's border samples exactly (153,191,255) against an authored (0.6,0.75,1.0), unblended — the untextured branch is bit-exact. The badge sprite is authored at (460,58), 64x64, and its gradient starts at x=460 with the clear colour at 455 — the RGBA-modulate branch, sampling a table slot. The two debug-line segments land on their computed screen coordinates. All three fragment branches, the pixel-to-NDC mapping, the top-left origin, straight-alpha blending and table sampling are therefore all confirmed on Vulkan, which is everything the offline GL gate confirms about the same code on GL.

The plan's V6 milestone is amended rather than claimed: "full game frame on Vulkan" is not reachable while V4c/V4d are parked and the world renderers and TextureCache are still raw GL, so V6 delivers the backend plus the two renderers that can use it today. The accumulated user-gate table gains a V6d row for the paperdoll/appraisal viewport sprite — the one retained-UI texture the offline scene never draws, on a slice that changed how every UI texture is sampled.

App tests 4,057 passed / 3 skipped, unchanged. Offline pixel gate against f6f58a12: differing fraction 3.20e-05, 18 pixels of 563,200, inside the documented 15-23 pixel noise band — as expected, since this commit touches only Vulkan files and the campaign doc.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/plans/2026-07-27-vulkan-campaign.md

@@ -511,6 +511,7 @@ session, rather than one at a time:
 | V4e | Particles (again) |
 | V4f | Sky — deliberately masked for determinism |
 | V4g | Paperdoll and appraisal viewports, portal transit |
+| V6d | The paperdoll/appraisal viewport sprite. It is the one retained-UI texture the gate's scene never draws, and V6d changed how every UI texture is sampled — from a bound texture unit to a table slot. The seam that registers it (`GlGpuDevice.RegisterExternalColorTexture`) is unchanged and its handle is now simply encoded rather than resolved back to a GL name, but that path is unproven by anything automated. Check it with the dungeon/portal pass above rather than on its own |
 
 **The user confirmed on 2026-07-27 that the local ACE server is always available
 and they will verify visually on request.** That converts this table from deferred
@@ -553,7 +554,7 @@ because sample positions are not specified across implementations.
 | **V4g** ↻ **re-sequenced — §5.5.5** | `PrivateEntityViewportRenderer` → `IGpuRenderTarget`; `PortalDepthMaskRenderer` + `PortalTunnelPresentation` → stencil/depth-mask pipelines. | pixel gate incl. paperdoll and portal transit |
 | **V4h** ↻ **re-sequenced — §5.5.5** | Frame-spine formalization: pass executors emit real declared `BeginPass`/`EndPass` (clears and framebuffer management move out of the spine and into pass load/store ops), flight/screenshot/resize/profiler move onto the RHI, `OpenGLGraphicsDevice`'s live role retires, Chorizite consumers are audited, and the architecture test lands. **Milestone: seam complete.** | pixel + connected lifecycle + R6 soak + complete Release suite + interim perf (RHI-on-GL CPU p50 ≤ 1.95 ms) |
 | **V5** ✅ | Vulkan bring-up, dark: `ACDREAM_RENDER_BACKEND`, surface/instance/device/queues/swapchain, the capability record/probe/guard with the exit-4 contract, a clear-colour loop with screenshot and clean shutdown. | VK boots to clear on the RX 9070 XT; forced-unsupported knob → exit 4 |
-| **V6** | Vulkan RHI backend, dark, three sequential commits: **a** allocator/buffers/staging/rings/timeline; **b** textures/BC mips/samplers/descriptor table/render targets/MSAA resolve; **c** `.spv` toolchain, pipelines, pipeline cache, negative viewport, push constants, timestamps, readback, debug names. **Milestone: full game frame on Vulkan.** | per-commit build + tests; VK renders world, UI, paperdoll, portals |
+| **V6** | Vulkan RHI backend, dark, four sequential commits: **a** ✅ allocator/buffers/staging/rings/timeline (`fb9c6693`); **b** ✅ textures/BC mips/samplers/descriptor table/render targets/MSAA resolve (`9eae4963`); **c** ✅ `.spv` toolchain, pipelines, pipeline cache, negative viewport, push constants, timestamps, readback, debug names (`234fe91d`); **d** ✅ first production renderers — `TextRenderer` and `DebugLineRenderer` on both backends, the colour-format contract amendment, and the retained UI drawn on Vulkan. **Milestone deferred:** "full game frame on Vulkan" is not reachable while V4c/V4d are parked and the world renderers plus `TextureCache` are still raw GL, so V6 delivers the backend and the two renderers that can use it today. | per-commit build + tests; V6d additionally pixel-gates GL and captures a Vulkan UI frame |
 | **V7** | GL-versus-Vulkan differential: `tools/run-backend-differential-gate.ps1`, strict paired-PNG compare, divergences fixed in the Vulkan backend only, then lifecycle + R6 soak natively on Vulkan, one validation-layer-clean run, one RenderDoc capture. **Milestone: parity.** | every differential checkpoint passes; both connected routes green on VK |
 | **V8** | Perf gate on the RX 9070 XT, uncapped, both backends, same route. | §2 acceptance table; parity is the floor |
 | **V9** | Linux + CI: X11/Wayland surfaces; a `linux-vulkan` job on lavapipe (probe accepts on a real 1.3 software device, a short real render under xvfb, forced-unsupported → exit 4, `.spv` freshness). Physical Linux GPU row deferred post-cutover, as for Slice L. | CI green including the new job |