cli: add compiler command and update e2e tests

[strimo378]

Motivation

• Provide a command-line entrypoint to compile ONNX models to C so the compiler can be used as a CLI tool.
• Make end-to-end tests exercise the public CLI instead of calling compiler internals to better verify real usage.
• Allow emitting a JSON testbench from the generated C to validate correctness in integration tests.

Description

• Add src/onnx2c/cli.py implementing onnx2c compile with options --template-dir, --model-name, and --emit-testbench and a main() function that returns an exit code.
• Add src/onnx2c/__main__.py to enable python -m onnx2c invocation.
• Update end-to-end tests in tests/test_endtoend_ops.py and tests/test_endtoend_features.py to invoke the CLI via python -m onnx2c compile ..., set PYTHONPATH so the local src is loadable, and consume the generated C/testbench outputs.
• Document CLI usage and testbench invocation in README.md with example python -m onnx2c compile path/to/model.onnx build/model.c commands.

Testing

• Ran pytest -q tests/test_endtoend_ops.py tests/test_endtoend_features.py which completed successfully.
• The test run reported 6 passed (all modified end-to-end tests passed).
• The tests compile the generated C with the system C compiler and execute the emitted JSON testbench to validate outputs.
• No additional automated test failures were observed during the rollout.

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Codex Task