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>

CLAUDE.md

@@ -572,6 +572,7 @@ render/streaming work at `claude-memory/project_render_pipeline_digest.md`.
 Documentation entry point: [`docs/README.md`](docs/README.md).
 
 For canonical state, read in this order:
+- [`docs/plans/2026-07-27-vulkan-campaign.md`](docs/plans/2026-07-27-vulkan-campaign.md) — **ACTIVE: Campaign V, OpenGL → Vulkan.** The pinned RHI contract, the Vulkan technical decisions, the V0–V11 slice table with per-slice gates, and the subagent execution rules. Read this before touching anything under `src/AcDream.App/Rendering/`. ImGui dev tools and UI Studio are NOT ported and are deleted at V11.
 - [`docs/plans/2026-05-12-milestones.md`](docs/plans/2026-05-12-milestones.md) — milestone targets + freeze list per milestone
 - [`docs/plans/2026-04-11-roadmap.md`](docs/plans/2026-04-11-roadmap.md) — what's shipped, what's in flight, what's next
 - [`docs/ISSUES.md`](docs/ISSUES.md) — open + recently closed bugs (tactical)