[CoreAI] Add the Core AI SDK Swift and Objective-C bridge

[metascroy]

Adds the private bridge between ExecuTorch's Objective-C++ code and the Swift CoreAI framework, and packages it for CMake consumers. No ExecuTorch delegate yet; a later PR wires this into CoreAIBackend.

• runtime/ETCoreAIBridge.h: Objective-C protocols for model acquisition, prepared models, sessions and tensor transfer, plus availability, device architecture and loader factory entry points.
• runtime/ETCoreAIModel.swift (module CoreAIBridge):
  • Restore models from bookmarks, specialize source bundles with the persistent default SDK cache, bind named functions with ordered inputs/outputs, and evict cache entries.
  • Asynchronous SDK calls run in detached tasks and report through completion blocks. Only an SDK nil result is a cache miss; thrown errors keep their underlying cause.
  • Inputs are borrowed from caller storage through Core AI raw views; outputs use one memcpy for dense row-major arrays and strided copies otherwise.
• runtime/ETCoreAITensor.mm, runtime/coreai_bridge.mm: tensor wrappers and C entry points, built as the private coreai_bridge_obj object library so the delegate can absorb them and host tests can substitute a fake bridge.
• Build (backends/apple/coreai/CMakeLists.txt): requires CMake 3.31 and Ninja/Xcode, keeps ExecuTorch's numeric preprocessor definitions away from swiftc, applies an OS 27 minimum only to Core AI targets, and builds coreai_swift. cmake/PublishSwiftHeader.cmake publishes the generated header for Xcode.
• Tests: runtime/test/SwiftBridgeTests.swift is built with the bridge sources into the coreai_swift_bridge_test XCTest bundle (Ninja, macOS) and run by CTest under xcrun xctest. The invalid-URL case uses a non-file coreai-test: URL.
• README: "SDK Bridge" and the build requirements in "Building".
• Changes outside backends/apple/coreai are CMake and CI only, and only take effect when EXECUTORCH_BUILD_COREAI=ON:
  • tools/cmake/ExecuTorchCoreAIDependencies.cmake (new): resolves Foundation, CoreAI and Swift runtime link paths from the consumer's SDK; fails configuration for SDKs older than 27.
  • CMakeLists.txt: install that file into the package config.
  • tools/cmake/preset/default.cmake: note the SDK 27 requirement.
  • .github/workflows/coreai.yml: also trigger on changes to ExecuTorchCoreAIDependencies.cmake.

Stack: 5 of 9, based on #23390. Review only this PR's commit. Next: #23392.

Test plan: built locally for macOS 27.0; ctest -N lists coreai_host_test and coreai_swift_bridge_test, which the Core AI workflow runs on the macOS 27 runner. The XCTest bundle links CoreAI, so it only runs there (first CI run of it). lintrunner is clean.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/23391

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❌ 1 New Failure, 7 Pending, 1 Unclassified Failure

As of commit 756e906 with merge base 0b3d26d ():

NEW FAILURE - The following job has failed:

• Apple / build-benchmark-app / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 128

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• Test ROCm AOTI / unittest-rocm-gfx950-rocm7.2 / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  backends/cuda/tests/test_sdpa_splitk_replacement.py::TestUnsupportedSDPAStaysWithRegularLowering::test_float_mask_is_not_replaced

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