physics: to_petgraph multigraph parity and dangling-edge-drop contracts are fixture-free

[tsondru]

Provenance: audit-sweep phase 1, WI-P08 (N-catgraph-physics-09, minor).

Both documented contracts (parallel-edge preservation; dangling-edge drop on merged targets) have no fixture.

Fix: a duplicate-parallel-edge fixture and a stale-target merge-edge fixture.

Routed: phase 3.

[tsondru]

Folded into #392 (coverage backlog, 2026-09-01 triage: test-only row, prod diff 0 expected). Reopen if a production defect surfaces here.