[wip] 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):

• -> [wip] Add CI test for CoreML LoRA multimethod export #18354
• [wip] Add --method argument to CoreML static LLM runner #18355
• Add LoRA multimethod export to CoreML static LLM export #18347
• Add skip_names and LoRALinear guard to replace_linear_with_split_linear #18346
• Add LoRA support to StaticAttention for split_mha=False #18345
• Refactor: extract _prepare_eager_model() from CoreML export main() #18343

[pytorch-bot]

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

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⏳ 43 Pending, 3 Unrelated Failures

As of commit b0228a9 with merge base dd7464a ():

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux-basic (mv3, xnnpack-quantization-delegation, cmake, linux.2xlarge, executorch-u... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-qnn-wheel-packages-linux (3.13) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-setup-linux-gcc / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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