fix: avoid subprocess in platform detection

[nightcityblade]

Fixes #1762

Summary

• replace platform.platform() with fork-free system(), release(), and machine() lookups
• preserve macOS, iOS, Android, Linux, and fallback platform classification
• add regression coverage that fails if platform.platform() is called

Testing

• uv run pytest tests/test_client.py -n0 -q (185 passed)
• ./scripts/lint (ruff, pyright, mypy, and import check passed)