Add LoRA multimethod export to CoreML static attention

[lucylq]

Add support for exporting LoRA adapters as separate methods in a CoreML
PTE file. CoreML POSITIONAL weight sharing deduplicates base weights
across methods so the binary overhead is just the lora_a/lora_b weights.

• StaticAttention: LoRA-aware projection creation for split_mha=False
• utils.py: skip_names + LoRALinear guard in replace_linear_with_split_linear
• export: --adapter CLI, adapter loading, _exclude_lora quantization filter,
  skip_split_names for POSITIONAL sharing, multimethod export branches

Authored with Claude.

[lucylq]

Stack from ghstack (oldest at bottom):

• -> Add LoRA multimethod export to CoreML static attention #18344
• Refactor: extract _transform_eager_model() from CoreML export main() #18343
• Update lora def #18342

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18344

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❌ 1 New Failure, 121 Pending

As of commit b891fba with merge base eb92cec ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner (gh)
  >>> Lint for examples/apple/coreml/llama/export_static_llm_coreml.py:

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