fix: handle regularized no-link networks

[antoneri]

Issue

Closes #360

Risk

high

Summary

• use a uniform prior in regularized flow calculation when the network has no links, instead of deriving prior weights from 0/0 degree statistics
• allow the existing one-level fallback to collapse prior-only regularized runs with zero links into a single top module
• add a Python regression covering both directed and undirected regularized no-link runs

Verification

• PATH="$PWD/.codex-bin:/opt/homebrew/bin:$PATH" CXXFLAGS="-I/opt/homebrew/opt/libomp/include" LDFLAGS="-L/opt/homebrew/opt/libomp/lib" make python
• PATH="$PWD/.codex-bin:/opt/homebrew/bin:$PATH" python -m pytest test/test_flow.py -k regularized_no_links
• PATH="/opt/homebrew/bin:$PATH" CXXFLAGS="-I/opt/homebrew/opt/libomp/include" LDFLAGS="-L/opt/homebrew/opt/libomp/lib" make -B

Residual Risk

• This changes regularized behavior in a core edge case, so other prior-only or zero-link paths should still be watched for unintended partition fallback changes.