codegen: switch testbench RNG to 64-bit

[strimo378]

Motivation

• Replace the libc rand()-based generator with a deterministic 64-bit RNG to improve portability and randomness quality in generated testbenches.
• Produce reproducible testbench inputs without relying on environment-specific rand() behavior.

Description

• Update the testbench template to use a 64-bit xorshift RNG with static uint64_t rng_state, rng_next_u64() and rng_next_float() scaled by 2^64.
• Replace emitted testbench include from <stdlib.h> to <stdint.h> in c_emitter.py so the generated code compiles with the new 64-bit types.
• Refresh the golden snapshot tests/golden/add_model_testbench.c to reflect the new 64-bit RNG helper and float generation.

Testing

• Ran the test suite with pytest -n auto -q and all tests passed.
• Test run result: 11 passed.

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Codex Task