Feat: Fallback logging

[DarkMatter-999]

Adds strict fallback observability for the Vulkan backend and introduces regression coverage for CPU fallback behavior.

Changes

• Added configurable strict fallback mode.
• Added fallback invocation counting.
• Added fallback reporting through the Python API and torch.vk.
• Preserved the existing fallback warning text.
• Added regression tests covering:
  • Permissive CPU fallback.
  • Strict fallback rejection.
  • Fallback count increments.
  • Legacy warning compatibility.

Testing

Validated with:

PYTHONPATH=build-venv .venv/bin/python tests/test_fallback.py --device vk:0

[softcookiepp]

looks good to meeeeee