feat(render): Campaign V slice V0 — pin the Vulkan-shaped RHI contract

Campaign V migrates the renderer from OpenGL 4.3+extensions to a single
Vulkan 1.3 backend on Windows x64 and Linux x64, then deletes the GL path.
Motivation is compatibility and efficiency, not rescue: mandatory
GL_ARB_bindless_texture is the exact floor that parked Slice L (Mesa
D3D12/llvmpipe lack it) while Vulkan descriptor indexing is core, and
per-frame data can be written straight into mapped memory rather than
copied through BufferSubData.

V0 pins the contract every later slice codes against. Nothing consumes it
yet, so this commit changes no runtime behavior.

The seam is a minimal Vulkan-shaped RHI implemented FIRST on GL. That
ordering is the point: the twelve renderers then port one at a time under a
strict pixel gate on the still-shipping backend, so a divergence is
attributed to one slice instead of surfacing at a big-bang integration.
Duplicating renderers per backend was rejected because WbDrawDispatcher is
4,449 lines holding only ~62 GL call sites — the API surface is small and
the retail-fidelity CPU logic is large, and forking the latter is how subtle
regressions enter.

Contract highlights:
  - GpuBindingModel pins set/binding numbers dual-legal for GL and Vulkan
    GLSL. Storage bindings 0-8 keep today's shader numbering; UBOs move to
    their own set, which resolves the binding=1 collision GL only tolerates
    because it keeps SSBO and UBO tables separate.
  - GpuRingAllocation is a ref struct replacing every per-frame
    BufferSubData; the compiler forbids outliving the owning frame.
  - GpuTextureSlot replaces bindless handles. Unassigned is a loud
    uint.MaxValue sentinel rather than a silent resolve to slot 0 — the
    failure mode behind the magenta 1x1 UI placeholder bug. Renderers
    needing a fallback take the device's really-registered default slot.
  - Renderers always speak GL winding/viewport conventions; the Vulkan
    backend compensates with a negative viewport height in exactly one
    mapping function.

Verified while writing the plan: acdream's cameras already build
[0,1]-NDC projections (PortalProjection.cs:12-13), which is Vulkan's
convention. No projection rework is needed and depth precision improves,
at the cost of shifted z-fight patterns — the one pre-approved divergence
class, registered per instance at V7.

Gate: Release build green; App suite 3,785 passed / 3 skipped (3,763
baseline plus 22 new contract tests). Note for later slices, recorded in
the plan: run the suite in Release. LandblockBuildOriginTests'
far-strip test asserts behavior that LandblockStreamer.cs:505 deliberately
turns into a loud Debug.Assert in Debug builds, so a Debug run shows one
pre-existing failure that is not a regression.

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

src/AcDream.App/Rendering/Gpu/GpuPassDescription.cs

@@ -0,0 +1,80 @@
+using System.Numerics;
+
+namespace AcDream.App.Rendering.Gpu;
+
+/// <summary>
+/// The colour attachment for a pass.
+/// </summary>
+/// <param name="Target">
+/// The offscreen target to render into, or null for the backbuffer. On GL null
+/// means framebuffer 0; on Vulkan it means the acquired swapchain image (or the
+/// multisampled scratch image that resolves into it when
+/// <paramref name="Store"/> is <see cref="GpuStoreOp.Resolve"/>).
+/// </param>
+/// <param name="Load">What happens to existing contents on entry.</param>
+/// <param name="Store">What happens to contents on exit.</param>
+/// <param name="ClearColor">Clear value used when <paramref name="Load"/> is <see cref="GpuLoadOp.Clear"/>.</param>
+internal readonly record struct GpuColorAttachment(
+    IGpuRenderTarget? Target,
+    GpuLoadOp Load,
+    GpuStoreOp Store,
+    Vector4 ClearColor);
+
+/// <summary>
+/// The depth/stencil attachment for a pass. Depth is transient in every acdream
+/// pass — nothing reads it after the frame — so <see cref="Store"/> is normally
+/// <see cref="GpuStoreOp.DontCare"/>, which lets Vulkan skip writing it back to
+/// memory entirely.
+/// </summary>
+/// <param name="Load">What happens to existing contents on entry.</param>
+/// <param name="Store">What happens to contents on exit.</param>
+/// <param name="ClearDepth">Depth clear value. acdream renders with NDC z in [0,1], so far = 1.</param>
+/// <param name="ClearStencil">Stencil clear value; #117's portal punch uses the stencil aspect.</param>
+internal readonly record struct GpuDepthAttachment(
+    GpuLoadOp Load,
+    GpuStoreOp Store,
+    float ClearDepth,
+    uint ClearStencil);
+
+/// <summary>
+/// One rendering pass: a set of attachments, their load/store behaviour, and the
+/// sample count every pipeline used inside must match.
+///
+/// GL has no such object — its "pass" is implicit in whatever framebuffer happens
+/// to be bound — so making passes explicit is the single largest structural change
+/// the RHI imposes on the existing renderers. The GL backend therefore accepts an
+/// ambient encoder during Campaign V slices V1..V4g (draws outside any declared
+/// pass, preserving today's behaviour) and slice V4h removes that relaxation once
+/// every renderer declares its passes.
+/// </summary>
+internal sealed record GpuPassDescription
+{
+    /// <summary>Stable identifier, surfaced as a debug label in captures.</summary>
+    public required string Name { get; init; }
+
+    /// <summary>The colour attachment. Required — acdream has no colour-less passes.</summary>
+    public required GpuColorAttachment Color { get; init; }
+
+    /// <summary>Depth/stencil attachment, or null for 2-D passes that need no depth.</summary>
+    public GpuDepthAttachment? Depth { get; init; }
+
+    /// <summary>Samples per pixel. Must equal <see cref="GpuPipelineDescription.SampleCount"/> of every pipeline bound inside.</summary>
+    public int SampleCount { get; init; } = 1;
+
+    /// <summary>Clears colour and depth to the standard frame-start values against the backbuffer.</summary>
+    public static GpuPassDescription BackbufferClear(string name, Vector4 clearColor, int sampleCount) => new()
+    {
+        Name = name,
+        Color = new GpuColorAttachment(
+            Target: null,
+            Load: GpuLoadOp.Clear,
+            Store: sampleCount > 1 ? GpuStoreOp.Resolve : GpuStoreOp.Store,
+            ClearColor: clearColor),
+        Depth = new GpuDepthAttachment(
+            Load: GpuLoadOp.Clear,
+            Store: GpuStoreOp.DontCare,
+            ClearDepth: 1f,
+            ClearStencil: 0),
+        SampleCount = sampleCount,
+    };
+}