fix(python): avoid userdata reference cycle in FUSE demo

[030611]

Fixes #386.

The FUSE demo passes the full io tuple as userdata. That tuple contains the same iovec wrapper being prepared, while the C++ iov retains userdata through completion. This creates a wrapper -> C++ iov -> tuple -> wrapper cycle across the Python/C++ boundary.

Pass the expected byte count as scalar userdata instead, preserving the existing completion check without retaining the wrapper. Also clarify the userdata lifetime contract in the binding documentation.

Validation:

• Python source compilation
• static red/green check for the iovec-containing userdata path
• git diff --check

I could not run the FUSE/RDMA path locally. This does not change usrbio runtime ownership and does not address the separate prepare() error path.