feat(render): V6e — the sky's uniforms become a buffer and its texture a table slot

Campaign V slice V6e, last of three. Sky was the hardest of the four pairs
because it was the only one that still worked the way a 2004 shader works: a
dozen loose uniforms pushed one glUniform call at a time, and a texture bound to
unit 0 with a sampler object chosen per submesh. Vulkan GLSL has neither a
default uniform block nor a way to declare a bare sampler, so both had to move —
and the second one had a sting in it.

The uniforms go into a `SkyParams` std140 block at uniform binding 4, the new
pre-authorized constant in GpuBindingModel (1, 2 and 3 are SceneLighting, the
terrain clip block and terrain tiling; the contract test now proves the three
constants and that literal 2 do not collide). Three matrices are 192 bytes on
their own, so the 96-byte push-constant block was never in the running. The
block's member order IS its layout: std140 aligns a vec3 to 16 bytes while using
12, so each of the three lighting vectors is followed by the float that rides in
its pad word, which is why colours and per-surface scalars interleave rather
than grouping by meaning. SkyParamsLayoutTests asserts all twelve offsets and
the 256-byte size, because getting one member wrong would read the sun direction
as a colour with no compile error, no link error and no GL error to say so.

The texture is the interesting half. sky.frag now reads through the shared table
(ACDREAM_SAMPLE_2D), and a bindless handle BAKES its sampler — so the
per-submesh Repeat-versus-ClampToEdge choice, which used to be a glBindSampler
on unit 0, becomes which slot the submesh asks for. SkyRenderer interns one
handle per (texture, wrap) pair, exactly as ManagedGLTextureArray has done since
the world path went bindless, and exactly the shape Vulkan's table has, where an
entry is a combined image sampler. Same two SamplerCache objects, same wrap
behaviour, consulted once at interning instead of once per draw. A pleasant
consequence: the sky no longer touches texture unit 0, so the load-bearing
`BindSampler(0, 0)` restore at the end of the pass — there because the binding
was global state that would otherwise force ClampToEdge on the next renderer —
has nothing left to undo and is gone.

Gates. Release build clean; App tests 4,072 passed / 3 skipped (4,057 baseline,
plus the sentinel guard from the previous commit and fourteen sky-layout
assertions). Offline pixel gate against 95f8c25f: 18 px of 563,200 compared
(3.20e-05), inside the documented 15–23 px band.

That gate masks the sky for determinism, so it proves nothing about this commit
and the sky renderer has no automated pixel coverage at all. What was done
instead: a base-versus-head offline capture at ALL SEVEN day groups, built by
stashing the change and rebuilding so the two runs differ only in this commit.
Every pair matches in gradient, cloud sheet, horizon band and fog — including
day group 2's salmon cloud band and day group 6's green one, which between them
exercise texture sampling, per-vertex tint, blend mode and fog. Then 3/3
RENDERED on the desktop-witness repeat-connected gate.

That bounds the risk; it does not close it. The offline camera is fixed and
looks down, so a thin band of dome is all it ever sees: the sun and moon
(additive, high) and the rain cylinder (the one sky mesh that surrounds the
camera, and the one whose REPEAT wrap is most visible) remain unproven. Recorded
as user-gate debt in §5.1 alongside V2c's and V4e's particles — check it by
standing outside at dawn or dusk, and by standing in rain.

Manifest: 8/9 pairs compile. `terrain_modern` is the last production pair, and
it is blocked on V4d's content rather than on dialect — details in §5.5's slice
table. `mesh` has no consumer.

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

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

@@ -512,6 +512,7 @@ session, rather than one at a time:
 | 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 |
+| V6e | **Particles, again — and now sky.** The particle half is V2c's and V4e's debt restated: the offline scene draws no particles, so nothing automated saw the varying retype or the ACDREAM_TEXTURE_NONE sentinel. The sky half is new and larger: V6e moved a dozen loose uniforms into a `SkyParams` uniform buffer and moved the sky's texture from a bound unit-0 texture-plus-sampler to a bindless (texture, wrap) table slot, and the gate masks the sky band for determinism. What WAS checked, and should be read as bounding the risk rather than closing it: a base-versus-head offline capture at **all seven day groups**, matching in gradient, cloud sheet, horizon band and fog on every one — including day group 2's salmon cloud band and day group 6's green band, which exercise texture sampling, tint, blend and fog together — plus 3/3 RENDERED on the desktop-witness repeat gate. **What remains unproven is pixel-exactness and the parts of the dome the fixed outdoor camera cannot see**: the sun and moon (additive surfaces high in the sky) and the rain cylinder, which is the one sky mesh that surrounds the camera and the one whose REPEAT wrap mode is most visible. Stand outside at dawn or dusk, and stand in rain |
 
 **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
@@ -554,7 +555,18 @@ 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, 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 |
+| **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; **e** ✅ every remaining production shader crosses the dialect — `mesh_modern` (`935f4dc3`), both particle pairs (`602bc9dd`) and `sky` — leaving 8/9 pairs compiling to SPIR-V. **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, the two renderers that can use it today, and the shaders the Vulkan world path will be built on. | per-commit build + tests; V6d additionally pixel-gates GL and captures a Vulkan UI frame; V6e pixel-gates GL per commit and adds a seven-day-group sky comparison plus a 3-run desktop-witness gate |
+
+**The one production pair still not Vulkan-expressible after V6e is `terrain_modern`**,
+blocked on exactly the two things V4d was going to do: `uView`/`uProjection` are
+two loose `mat4` uniforms (128 bytes — they cannot both fit the 96-byte push
+block, which is why V4d's first sub-commit converged them into one
+`uViewProjection` on its own pixel gate), and `uTexTiling[36]` is the 144-byte
+array `UniformTerrainTiling` was reserved for. V6e left it alone because
+converging the matrices moves a multiply from per-vertex GPU to a CPU multiply —
+a real numeric change that the plan requires be attributable on its own gate,
+and one that belongs to whoever re-lands V4d's content rather than to a shader
+dialect slice. `mesh` is a tenth pair with no consumer at all; see the V6e report.
 | **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 |