build(bazel): add experimental darwin arm64 platform

Registers the zig darwin cc toolchain, an aarch64-apple-darwin rust
toolchain, and a //:darwin_arm64 platform. Toolchain resolution works
and pure-Rust rlibs cross-compile from Linux; crates whose C
dependencies do not build for mach-o (aws-lc-sys) and anything linking
Apple frameworks still fail — real macOS artifacts stay on Apple
hardware for now.

Signed-off-by: Simon Scatton <sscatton@nvidia.com>

Author: SDAChess

BUILD.bazel

@@ -12,3 +12,11 @@ platform(
     name = "linux_arm64_musl",
     parents = ["@zig_sdk//libc_aware/platform:linux_arm64_musl"],
 )
+
+# Experimental: cross-compiling library crates to macOS. zig provides the
+# darwin libc headers and libSystem stubs; targets needing Apple framework
+# link stubs (Security, CoreFoundation) cannot link without an Apple SDK.
+platform(
+    name = "darwin_arm64",
+    parents = ["@zig_sdk//platform:darwin_arm64"],
+)

MODULE.bazel

@@ -46,6 +46,19 @@ rust.repository_set(
     ],
 )
 
+# Experimental macOS cross-compilation (library crates only; linking needs
+# Apple framework stubs that zig does not ship).
+rust.repository_set(
+    name = "rust_darwin_arm64",
+    exec_triple = "x86_64-unknown-linux-gnu",
+    target_triple = "aarch64-apple-darwin",
+    versions = ["1.95.0"],
+    target_compatible_with = [
+        "@platforms//os:macos",
+        "@platforms//cpu:aarch64",
+    ],
+)
+
 use_repo(rust, "rust_toolchains")
 
 # The musl hub entries must be registered before //:all — the gnu entries in
@@ -76,6 +89,7 @@ use_repo(hermetic_toolchain, "zig_sdk")
 register_toolchains(
     "@zig_sdk//libc_aware/toolchain:linux_amd64_musl",
     "@zig_sdk//libc_aware/toolchain:linux_arm64_musl",
+    "@zig_sdk//toolchain:darwin_arm64",
 )
 
 # Hermetic protoc for openshell-core's build script, injected via $PROTOC