Validate topology graphs before submit

[bnmajor]

Validate before submission rather than allowing CODES to fail at startup.

• Switch names unique within a topology.
• Link source and target both exist; no self-links; no duplicate directed link (A→B twice).
• terminals >= 0; bandwidths and switch_buffer positive.
• Reachability: every terminal pair that traffic can be generated between must have a next hop (see compute_routes()).
• Raise errors on the form, not as a failed run.