Add a CoreML rung on macOS

The macOS ladder was the CPU provider alone, with CoreML listed as a gap.
It is now CoreML, then the CPU, then tract — unmeasured, since nobody here
has a Mac, and safe to ship unmeasured because the probe's clock rejects a
CoreML slower than the CPU and `attempt` refuses one that crashes.

- `Rung::CoreMl`, a compiling rung like TensorRT: an ML Program with every
  compute unit allowed, falling back to the CPU until each model's program
  is built. The embedder stays on the CPU, as on the Hexagon (§7).
- The cache is one directory per model and runtime version. CoreML keys a
  model committed from memory on its input and node names, not its
  weights (ONNX Runtime 1.29, coreml_execution_provider.cc), so two
  exports of one architecture would otherwise share a program.
- The fingerprint on macOS is the chip and the OS release, which ships
  CoreML.
- The desktop looks for the runtime in the bundle's Contents/Frameworks
  and Homebrew's prefixes; fetch-desktop-runtime.sh on a Mac downloads
  ONNX Runtime 1.29.0 for Apple silicon, which carries CoreML.

docs/dev/macos.md says what exists, how to build it, and which log lines
to ask a Mac user for.

core/dr-inference-engine/src/engines.rs

@@ -44,6 +44,20 @@ pub fn context_path(cfg: &Config, bytes: &[u8]) -> PathBuf {
         .join(format!("{:016x}_ctx.onnx", hash(bytes)))
 }
 
+/// Where CoreML compiles `bytes` to: one directory per model, because
+/// CoreML's own cache key leaves out the weights of a model loaded from
+/// memory (`session::coreml`), and one per runtime version, which wrote it.
+pub fn coreml_dir(cfg: &Config, bytes: &[u8]) -> PathBuf {
+    let runtime = match crate::api::runtime() {
+        crate::Runtime::OnnxRuntime { version, .. } => version,
+        crate::Runtime::Tract => "tract".into(),
+    };
+    cfg.cache_dir
+        .join("coreml")
+        .join(runtime)
+        .join(format!("{:016x}", hash(bytes)))
+}
+
 /// After the probe: compile every configured model the selected rung can
 /// take, smallest first, recording each as it lands.
 pub fn run() {