Update README with Nabla HLSL Builtin Library details

include/nbl/builtin/hlsl/README.md

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+# Nabla HLSL Builtin Library
+
+## Overview
+
+Shared shader code for Nabla. Many headers compile both as HLSL on the GPU and
+as C++ on the host. The two paths are split by the `__HLSL_VERSION` macro: the
+host branch often forwards to the C++ standard library, while the device branch
+provides the GPU equivalent, often backed by SPIR-V intrinsics. `bit.hlsl` is a
+small example of both branches in one file.
+
+All code lives in the `nbl::hlsl` namespace.
+
+## Single-source HLSL/C++
+
+This folder is Nabla's shader-side support library: reusable HLSL headers for
+math, types, intrinsics, algorithms, compatibility helpers, and graphics code.
+The same header can be compiled by the shader compiler for GPU use and by the
+C++ compiler for host-side tests, tools, and shared data definitions.
+
+The compatibility layer is what makes that possible. `cpp_compat.hlsl` and the
+`cpp_compat/` folder provide the C++ side of HLSL types and intrinsics, while
+other headers use `__HLSL_VERSION` to choose between host implementations
+(`std`, C++ helpers, or test code) and device implementations (HLSL builtins,
+GLSL-style compatibility wrappers, or SPIR-V intrinsics).
+
+This lets Nabla write shader abstractions in HLSL while keeping them close
+enough to C++ to share structures, validate behavior on the host, and build
+shader utilities similar to parts of the C++ standard library.
+
+For more context, see the DevSH presentations:
+
+- https://www.youtube.com/watch?v=JCJ35dlZJb4
+- https://www.devsh.eu/presentations
+
+## Placement rules
+
+Root files that share names with C++ standard library headers are for the
+matching `std::` counterpart only. For example, `bit.hlsl` is the `<bit>`
+counterpart and should stay limited to things like `bit_cast`, `rotl`, `rotr`,
+and `countl_zero`.
+
+Code beyond the standard library goes in a subfolder, even if it is related to a
+root file. The bitfield abstraction is not part of `<bit>`, so it belongs in
+`utils/bitfield.hlsl`, not in `bit.hlsl`.
+
+Prefer folder names that match their namespaces, but do not force it when the
+result reads badly. `utils/bitfield.hlsl` is better than
+`bitfield/bitfield.hlsl` if the second form pushes toward
+`hlsl::bitfield::bitfield`.
+
+## How to extend it
+
+1. If it mirrors a C++ standard library header, put it in the matching root
+   file.
+2. If it is a Nabla extension or helper, put it in a subfolder.
+3. Reuse an existing folder when it fits. Add a new one only when needed.
+
+## Structure
+
+### Root files
+
+The "std" column marks files that are the GPU counterpart of a C++ standard
+library header. Those hold only what the standard header provides.
+
+| File | std | Contents and what belongs here |
+| --- | --- | --- |
+| `macros.h` | | Core preprocessor macros, including the `static_assert`/`assert` shims for HLSL. Cross-compilation macros go here. |
+| `cpp_compat.hlsl` | | Umbrella include for the C++ compatibility layer (pulls in `cpp_compat/`). |
+| `algorithm.hlsl` | `<algorithm>` | `swap` and other standard algorithms. |
+| `bit.hlsl` | `<bit>` | `bit_cast`, `rotl`, `rotr`, `countl_zero`. |
+| `complex.hlsl` | `<complex>` | `complex_t`. |
+| `concepts.hlsl` | `<concepts>` | Entry point for the concepts library (includes `concepts/`). |
+| `functional.hlsl` | `<functional>` | `reference_wrapper` and function objects. |
+| `limits.hlsl` | `<limits>` | `numeric_limits`. |
+| `memory.hlsl` | `<memory>` | Pointer and reference helpers, e.g. `pointer_to` for BDA refs. |
+| `numbers.hlsl` | `<numbers>` | Math constants (`e`, `pi`, ...). |
+| `tuple.hlsl` | `<tuple>` | `tuple`. |
+| `type_traits.hlsl` | `<type_traits>` | Type trait structs and aliases. |
+| `utility.hlsl` | `<utility>` | Standard utilities (currently `declval`). |
+| `tgmath.hlsl` | `<tgmath>`/`<cmath>` | Type-generic math functions. |
+| `ieee754.hlsl` | | IEEE-754 float layout traits and bit helpers. |
+| `mpl.hlsl` | | Template metaprogramming helpers (compile-time, boost::mpl style). |
+| `enums.hlsl` | | Engine enums shared host and device (e.g. `ShaderStage`). |
+| `format.hlsl` | | Texel block format enum and format pack/unpack (includes `format/`). |
+| `colorspace.hlsl` | | Colorspace conversions entry point (includes `colorspace/`). |
+| `morton.hlsl` | | Morton / Z-order code encode and decode. |
+| `acceleration_structures.hlsl` | | Ray tracing acceleration structure build structs. |
+| `indirect_commands.hlsl` | | Indirect draw and dispatch command structs. |
+| `binding_info.hlsl` | | Compile-time descriptor binding info structs. |
+| `device_capabilities_traits.hlsl` | | Traits to query device capabilities at compile time. |
+| `array_accessors.hlsl` | | Generic array `get`/`set` accessor structs. |
+| `memory_accessor.hlsl` | | Accessor wrappers over memory with atomic and barrier method detection. |
+| `member_test_macros.hlsl` | | Macros to detect presence of struct members and methods. |
+| `ndarray_addressing.hlsl` | | Multi-dimensional to linear index addressing. |
+| `scanning_append.hlsl` | | Result types for the scan-and-append primitive. |
+| `surface_transform.h` | | Swapchain surface transform flags and helpers. |
+
+### Subfolders
+
+| Folder | Contents and what belongs here |
+| --- | --- |
+| `barycentric/` | Barycentric coordinate utilities. |
+| `bda/` | Buffer Device Address: typed pointers, references, and accessors. |
+| `blit/` | Image blit and normalization compute shaders and their parameters. |
+| `bxdf/` | BxDF models: fresnel, NDF, reflection, transmission. |
+| `colorspace/` | Transfer functions (EOTF/OETF) and CIEXYZ encode/decode. |
+| `concepts/` | Concept definitions (core, vector, matrix, accessors). |
+| `cpp_compat/` | The C++/HLSL compatibility layer: vector, matrix, intrinsics, promote, truncate. |
+| `emulated/` | Software-emulated types (`float64_t`, `int64_t`, emulated vector/matrix) for platforms without native support. |
+| `ext/` | Helpers tied to engine extensions (e.g. FullScreenTriangle). |
+| `fft/` | FFT building blocks. See [`fft/README.md`](fft/README.md). |
+| `format/` | Pixel/texel format pack and unpack (octahedral, shared exponent). |
+| `glsl_compat/` | GLSL builtin equivalents (core, subgroup ops). |
+| `ieee754/` | IEEE-754 implementation details. |
+| `ies/` | IES light profile sampling and textures. |
+| `math/` | Math routines: geometry, linalg, quaternions, equations, quadrature, and more. |
+| `matrix_utils/` | Matrix traits, compile-time and runtime. |
+| `path_tracing/` | Path tracing building blocks: ray gen, accumulators, integrators. |
+| `portable/` | Type aliases that pick native or emulated types per platform. |
+| `prefix_sum_blur/` | Prefix-sum based blur. |
+| `random/` | RNGs (lcg, pcg, tea, xoroshiro) and adaptors. |
+| `rwmc/` | Reweighted Monte Carlo: cascade accumulator, resolve, splatting. |
+| `sampling/` | Sampling distributions and warps (alias table, spherical shapes, mappings). |
+| `scan/` | Device-wide scan (prefix sum) primitives and schedulers. |
+| `shapes/` | Geometric primitives (aabb, obb, line, triangle, beziers, ...). |
+| `sort/` | Sorting primitives (counting sort). |
+| `spirv_intrinsics/` | Raw SPIR-V intrinsic declarations. |
+| `subgroup/` | Subgroup-level collectives (ballot, arithmetic, basic, fft). |
+| `subgroup2/` | Newer subgroup collective API. |
+| `testing/` | Comparison helpers for tests (approx compare, max error). |
+| `text_rendering/` | Text rendering (MSDF). |
+| `tgmath/` | `tgmath` implementation details (isnan, output structs). |
+| `vector_utils/` | Vector traits. |
+| `visualization/` | Visualization helpers (turbo colormap). |
+| `workgroup/` | Workgroup-level collectives (arithmetic, ballot, scan, shuffle, fft). |
+| `workgroup2/` | Newer workgroup collective API. |