docs(readme): convert README.adoc -> Markdown README.md

README must be real Markdown to render in GitHub community-health, the GitHub
profile, and external MCP directories (Glama) — AsciiDoc shows as raw markup
there. pandoc asciidoc->GFM, badges fixed to clickable, SPDX header kept as an
HTML comment, duplicate README.adoc removed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -1,75 +1,75 @@
-[![Sponsor](https://img.shields.io/badge/Sponsor-%E2%9D%A4-pink?logo=github)](https://github.com/sponsors/hyperpolymath)
+<!--
+SPDX-License-Identifier: CC-BY-SA-4.0
+SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+-->
 
-// SPDX-License-Identifier: CC-BY-SA-4.0
-// SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
-
-= AcceleratorGate.jl
-Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+Jonathan D.A. Jewell \<[j.d.a.jewell@open.ac](j.d.a.jewell@open.ac).uk\>
 :toc:
 
-image:https://img.shields.io/badge/License-MPL--2.0-blue.svg[License: MPL-2.0,link="https://www.mozilla.org/MPL/2.0/"]
+[![License: MPL-2.0](https://img.shields.io/badge/License-MPL--2.0-blue.svg)](https://www.mozilla.org/MPL/2.0/)
 
-== What this is
+# What this is
 
-AcceleratorGate.jl is a small, dependency-light *backend-selection layer*:
-a type hierarchy of compute backends (`JuliaBackend`, `CUDABackend`,
-`ROCmBackend`, `MetalBackend`, `TPUBackend`, `NPUBackend`, `FPGABackend`,
-`QPUBackend`, `DSPBackend`, …), environment-based availability detection,
-a platform probe, and a heuristic `select_backend` / `estimate_cost` /
-`fits_on_device` API.
+AcceleratorGate.jl is a small, dependency-light **backend-selection
+layer**: a type hierarchy of compute backends (`JuliaBackend`,
+`CUDABackend`, `ROCmBackend`, `MetalBackend`, `TPUBackend`,
+`NPUBackend`, `FPGABackend`, `QPUBackend`, `DSPBackend`, …),
+environment-based availability detection, a platform probe, and a
+heuristic `select_backend` / `estimate_cost` / `fits_on_device` API.
 
-It was extracted from `Axiom.jl`'s backend module to remove duplicated
-selection logic shared by a few packages in the same author's ecosystem.
+It was extracted from `Axiom.jl`’s backend module to remove duplicated
+selection logic shared by a few packages in the same author’s ecosystem.
 
-== What this is NOT (honest scope)
+# What this is NOT (honest scope)
 
 It does **not** execute GPU/accelerator computation. It contains no
-kernels and links no vendor runtimes. It is purely the *typing and
-selection* layer — the decision "which backend should this run on, and
-does it fit" — leaving execution to the real acceleration packages below.
+kernels and links no vendor runtimes. It is purely the **typing and
+selection** layer — the decision "which backend should this run on, and
+does it fit" — leaving execution to the real acceleration packages
+below.
 
-== Prior art
+# Prior art
 
 The Julia ecosystem already covers accelerated compute thoroughly:
 
-- *Actual acceleration*: `CUDA.jl`, `AMDGPU.jl`, `Metal.jl`, `oneAPI.jl`
-  — vendor GPU runtimes and kernels.
-- *Backend-agnostic kernels / array adaption*: `KernelAbstractions.jl`,
-  `GPUArrays.jl`, `Adapt.jl` — write-once-run-on-any-backend kernels and
-  array wrapping. These overlap conceptually with AcceleratorGate's
-  selection idea and are more mature for kernel portability.
+- **Actual acceleration**: `CUDA.jl`, `AMDGPU.jl`, `Metal.jl`,
+  `oneAPI.jl` — vendor GPU runtimes and kernels.
+
+- **Backend-agnostic kernels / array adaption**:
+  `KernelAbstractions.jl`, `GPUArrays.jl`, `Adapt.jl` —
+  write-once-run-on-any-backend kernels and array wrapping. These
+  overlap conceptually with AcceleratorGate’s selection idea and are
+  more mature for kernel portability.
 
 AcceleratorGate is deliberately narrower than all of the above: a tiny
 type/selection seam with no compute and minimal deps. If you need to run
 work on a GPU, use the packages above; AcceleratorGate only helps a host
-package *decide* and *describe* backends uniformly.
+package **decide** and **describe** backends uniformly.
 
-== Installation
+# Installation
 
-[source,julia]
-----
+```julia
 using Pkg
 Pkg.add(url="https://github.com/hyperpolymath/AcceleratorGate.jl")
-----
+```
 
-== Usage
+# Usage
 
-[source,julia]
-----
+```julia
 using AcceleratorGate
 
 select_backend(:matmul, 1_000_000)            # => JuliaBackend()  (no accelerator present)
 fits_on_device(JuliaBackend(), 10^6)          # => false
 estimate_cost(JuliaBackend(), :matmul, 1000)  # => 1000.0
 detect_platform()                             # => PlatformInfo(:linux, :x86_64, false, false, false, v"1.12", 64, :little)
 capability_report()                           # => Dict: platform, per-backend availability, strategy order
-----
+```
 
 (Output above is from an actual run on a CPU-only Linux host with no
-accelerators installed — backends report `available => false` and
+accelerators installed — backends report `available` `⇒` `false` and
 selection falls back to `JuliaBackend`.)
 
-== License
+# License
 
 MPL-2.0 (OSI-approved). See `LICENSE`, `LICENSES/MPL-2.0.txt`, and
 `REUSE.toml`.