C8 x C4 two-orbit obstruction at DSRG(64,13,6,1,3)

Scope: a semiregular C8 x C4 group with two orbits only.
No unrestricted nonexistence result is claimed.

Run with Python3 and assertions enabled:
  python3 verify_biabelian_C8xC4.py --out audit.json
  python3 biabelian_trace_census.py --out discovery.json
The first checker is independent standard-library exhaustive enumeration;
the second is the discovery DFS. Both produce40degree12+16degree16traces.
They list the same trace sets but use different orbit representative orders.
The independent checker also verifies a genuine directed8 control and its
complement, and exact rational nullspaces excluding all5traceorbits.

See proof.md for the mathematical reduction and existing-work credit.
Code is released under the MIT license; mathematical notes CC BY4.0.
