tests: portable popcount in arclab_init_hadamard (MSVC build)

[professorpalmer]

Overview

tests/test-backend-ops.cpp does not compile with MSVC since #356: arclab_init_hadamard uses __builtin_popcountll, which is GCC/Clang only (error C3861: '__builtin_popcountll': identifier not found). This replaces it with a small portable popcount (clear the lowest set bit until zero). The Hadamard sign is the parity of r & i, so the values are unchanged.

Additional information

Tested on Windows, MSVC 2022, CUDA 13, RTX 4070: test-backend-ops builds; -o FLASH_ATTN_EXT -b CUDA0 4335/4335 passed.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES. An AI agent found the build error and wrote the replacement under my direction; I reviewed the change (a standard bit-count loop) and the test run.