feat(render3d): gpu.ludic - the seam for a second graphics API, and what the GPU can do

Render state (depth, blending, culling, colour writes, alpha-to-coverage, depth bias,
scissor) and uniforms go through gpu_* / u_* and nowhere else; OpenGL state is cached and
only changes reach the driver. Frames are bit-identical to before at the fixed viewpoints.
R3D_GFX=gl|vk or gpu_request chooses a backend, falling back to OpenGL with a reason.

gpu_caps_probe() asks Vulkan, on Windows, for the 1.3 floor, ray tracing, mesh shaders, the
NVIDIA RTX generation, Reflex and HDR colour spaces, for a game's settings to grey out what
a machine cannot use; R3D_CAPS pretends to be a given card for tests.

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

changes/render3d-gpu-layer.md

@@ -0,0 +1,26 @@
+bump: minor
+type: feat
+Vulkan for Ludic, and the start of a second renderer beside OpenGL.
+
+- **`Vk.*`**: every command of Vulkan 1.0-1.4 and of the extensions a modern renderer is
+  built around (swapchain, HDR colour spaces, ray query and acceleration structures, opacity
+  micromaps, mesh shaders, variable rate shading, memory budget, pipeline libraries, NVIDIA
+  low latency, portability for MoltenVK), with every constant and every struct's size
+  (`<Struct>_sizeof`) and field offsets (`<Struct>_<field>`). Generated from the Vulkan
+  registry by **`ludic-dev vkgen`** into `runtime/native/vk_api.ludic` and `vk_thunks.ll`;
+  structs are plain memory filled by name with `Vk.put_i32` / `put_i64` / `put_ptr`. Every
+  size and offset was checked against the SDK's C headers (3128 facts). A C float is float
+  bits in an int; 64-bit values and non-dispatchable handles are `long`.
+- **The loader is opened at run time**, never linked: `vk_win.ll` (`vulkan-1.dll`) and
+  `vk_mac.ll` (`libvulkan.1.dylib`, MoltenVK). `Vk.open()` returning 0 means no Vulkan, and
+  the program carries on. `ludicc` and `ludic build` link both files for any program that
+  uses `Vk.*`.
+- `examples/rendering/vk_probe.ludic` reports what a machine's Vulkan can do;
+  `vk_compute.ludic` runs a Slang compute shader (`vk_compute.slang`) and reads the picture
+  back - on an RTX 3070 Ti and on an M4 Pro through MoltenVK, clean under the validation layer.
+- **render3d `gpu.ludic`**: the seam between the renderer and a graphics API. Render state
+  and uniforms go through `gpu_*` / `u_*` and nowhere else (OpenGL frames unchanged);
+  `R3D_GFX=gl|vk` or `gpu_request` chooses a backend, falling back to OpenGL with a reason;
+  `gpu_caps_probe()` detects, on Windows, the Vulkan 1.3 floor, ray tracing, mesh shaders,
+  the NVIDIA RTX generation, Reflex and HDR, for a settings screen to grey out what a machine
+  cannot use (`R3D_CAPS=rtx50|rtx40|rtx30|amd|intel|none` pretends, for tests).