Add CI test for CoreML LoRA multimethod export

[lucylq]

Tests three export configurations with file size validation:

1. Base only (single method) - baseline size
2. Base + LoRA adapter (multimethod) - small overhead from lora_a/lora_b
3. Base + LoRA + multifunction - same overhead (POSITIONAL sharing)

Uses stories110M with a synthetic zero-initialized LoRA adapter so
base and adapter outputs match. Inference tests run on macOS only.

Authored with Claude.

[lucylq]

Stack from ghstack (oldest at bottom):

• -> Add CI test for CoreML LoRA multimethod export #18354
• Add --method argument to CoreML static LLM runner #18355
• Add LoRA multimethod export to CoreML static LLM export #18347

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18354

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❌ 1 New Failure, 5 Cancelled Jobs, 3 Unrelated Failures

As of commit 1b2b985 with merge base e90d3c8 ():

NEW FAILURE - The following job has failed:

• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t c76f22b48e9d957f04d46276055bae625bdadf00ad8ab7487e216018c0b3415b /exec failed with exit code 1

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

• Test ARM Backend / test-arm / package-golden-artifacts (gh)
• Test ARM Backend / test-arm / test-backend-linux (arm_tosa_fp, models) / linux-job (gh)
• Test ARM Backend / test-arm / test-backend-linux (arm_tosa_fp, operators) / linux-job (gh)
• Test ARM Backend / test-arm / test-backend-linux (arm_vgf_fp, models) / linux-job (gh)
• Test ARM Backend / test-arm / test-backend-linux (arm_vgf_fp, operators) / linux-job (gh)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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