[CoreAI] Integrate the delegate execute path

[metascroy]

Wires the Core AI bridge into an ExecuTorch BackendInterface so CoreAIBackend can load and run models (runtime/coreai_backend.mm).

• init parses the manifest, checks the OS floor, selects the platform/architecture assets, acquires a prepared model through the load coordinator and binds the named function with ordered inputs/outputs.
• execute validates dtype (FP16/FP32), CPU storage, default dim order and contiguity, borrows input bytes without copying, then validates and resizes every output before copying any results.
• init and execute block their calling thread on Swift concurrency work, so the README tells callers not to invoke them from Swift async code.
• Build: backends/apple/coreai/CMakeLists.txt builds coreaidelegate (static, or shared against executorch_shared) from the backend sources plus coreai_bridge_obj, linking coreai_swift with the C++ linker so ExecuTorch's C/C++ options never reach swiftc.
• Tests:
  • The fake bridge gains function binding, sessions, execution and the C entry points the backend calls, so the real backend runs in coreai_host_test (delegate suite).
  • coreai_runtime_smoke checks final linkage, registration retention and availability. With EXECUTORCH_BUILD_TESTS=ON it is built and registered with CTest; otherwise it stays out of the default build.
• README: execution semantics, device-architecture selection, the default assets root, "Runtime options", and delegate linkage in "Building".
• Changes outside backends/apple/coreai are CMake only and only take effect when EXECUTORCH_BUILD_COREAI=ON:
  • tools/cmake/preset/default.cmake: EXECUTORCH_BUILD_COREAI requires the data loader extension.
  • CMakeLists.txt: add coreaidelegate to the backend list.

Stack: 7 of 9, based on #23392. Review only this PR's commit. Next: #23394.

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

[pytorch-bot]

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❌ 8 Cancelled Jobs, 3 Pending, 3 Unclassified Failures

As of commit a6f16ff with merge base 0b3d26d ():

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

• Cadence Build & Test / hifi-build / inputs.backend (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• Cadence Build & Test / vision-build / inputs.backend (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• 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

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Cadence Build & Test / hifi-build / Resolve CI docker image / resolve (gh)
• Cadence Build & Test / Resolve CI docker image / resolve (gh)
• Cadence Build & Test / vision-build / Resolve CI docker image / resolve (gh)
• Test CoreML Backend / Get changed files / get-changed-files (gh)
• Test NXP Backend / Get changed files / get-changed-files (gh)
• Test QNN Backend / test-qnn / package-golden-artifacts (gh)
• Test XNNPACK Backend / test-xnnpack / package-golden-artifacts (gh)
• Test XNNPACK Backend / test-xnnpack / Resolve CI docker image / resolve (gh)

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