codegen: emit nested JSON arrays in testbench

[strimo378]

Motivation

• Emit N-dimensional (ndim) JSON arrays for tensor data in the generated C testbench instead of flat arrays.
• Make testbench emission opt-in so callers can request nested JSON output via compiler options.
• Produce deterministic, template-driven testbench output that is easy to review and compare with golden files.

Description

• Add emit_testbench: bool to CompilerOptions and thread it into CEmitter.emit_model to enable optional testbench emission.
• Compute per-tensor loop metadata and linear index expressions with new helpers (_element_count, _index_expr) and include loop_vars, loop_indents, inner_indent, and rank in the template context.
• Replace the testbench template with templates/testbench.c.j2 that renders nested JSON arrays for inputs and outputs using the loop metadata, and include #include <stdio.h>/#include <stdlib.h> only when emit_testbench is enabled.
• Refresh the golden output tests/golden/add_model_testbench.c and add the test test_codegen_includes_testbench in tests/test_add_mvp.py which exercises CompilerOptions(template_dir=Path("templates"), emit_testbench=True).

Testing

• Ran the test suite with pytest -n auto -q and all tests passed (11 passed).
• The new golden comparison test test_codegen_includes_testbench executed as part of the suite and succeeded against tests/golden/add_model_testbench.c.
• The updated template was exercised by compiling the sample add model and writing the refreshed golden file before running tests.

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Codex Task