[CoreAI] Parse the delegate manifest at runtime and run host tests in CI

[metascroy]

Adds the native parser the Core AI runtime uses to read the delegate manifest, and builds and runs the backend's host tests in the Core AI workflow. This is the first of the runtime PRs; it does not add an ExecuTorch delegate yet.

• Runtime (runtime/coreai_assets.{h,mm}):
  • Parse and validate the manifest for inline source and aot_compiled_inline assets: function, ordered input_names/output_names, the files size map and bundle_digests.
  • Select AOT assets by exact platform and Core AI architecture match, with no fallback.
• Tests: a Foundation-only coreai_host_test GoogleTest executable (no Swift, no Core AI SDK) for the parser and asset selection. Path and file-map checks are covered because they feed filesystem writes; the manifest is otherwise treated as trusted exporter output.
• README: the "Runtime" manifest description, AOT architecture selection and "Host Tests".
• backends/apple/coreai/.clang-format: the root style without its Language: Cpp key, so clang-format also formats the backend's Objective-C headers.
• Changes outside backends/apple/coreai are CMake and CI only, and nothing changes unless EXECUTORCH_BUILD_COREAI=ON:
  • tools/cmake/preset/default.cmake: add EXECUTORCH_BUILD_COREAI (default OFF).
  • CMakeLists.txt: add the backend subdirectory under that option.
  • .github/workflows/coreai.yml: build backends/apple/coreai/all for macOS 27 with tests on, and run every CTest test the backend registers (--no-tests=error).

Stack: 1 of 9 Core AI runtime PRs, based on main. Next: #23388.

Test plan: built locally for macOS 27.0 (backends/apple/coreai/all, EXECUTORCH_BUILD_TESTS=ON); ctest -N lists coreai_host_test, which the Core AI workflow runs on the macOS 27 runner. lintrunner is clean.

[pytorch-bot]

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❌ 1 New Failure, 1 Unclassified Failure

As of commit 8bdd47d with merge base 0b3d26d ():

NEW FAILURE - The following job has failed:

• pull / test-vulkan-operators-linux / linux-job (gh)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1

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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