docs: the Python reference runtime has shipped, not underway

[Mohammed Alkindi (MohammedAlkindi)]

README.md:46 and docs/index.md:34 both still say "there is a Python-only reference runtime for ONNX underway that will also be supported". It is not underway, it shipped.

onnxscript/_internal/evaluator.py:529 defines class OnnxReferenceRuntimeEvaluator(BaseEvaluator), whose _eval runs onnx.reference.ReferenceEvaluator. It is public API, listed in onnxscript/evaluator.py __all__ at line 22 next to ORTEvaluator, and tests/eager_mode_test.py parametrizes the eager-mode tests across both evaluators.

So the two places a new user reads first are the only places saying the feature does not exist yet.

I reworded both to say ONNX Runtime is the default and the reference runtime is available as onnxscript.evaluator.OnnxReferenceRuntimeEvaluator, and left the surrounding bullets alone. docs/index.md keeps its {{ onnxscript }} substitution.

Running python docs/update_readme.py after the edit produces no further change, so the update_readme job's git diff --exit-code -- README.md still passes. My change is in the Design Overview prose, not the generated update-readme code stub.

Same shape as #3012, which corrected a check_model claim to match the code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.73%. Comparing base (e33b96c) to head (45d0347).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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##             main    #3048   +/-   ##
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  Coverage   72.73%   72.73%           
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  Files         265      265           
  Lines       32358    32358           
  Branches     3069     3069           
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  Hits        23536    23536           
  Misses       7779     7779           
  Partials     1043     1043           

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