physics: wasserstein identity/symmetry asserted on one uniform 3-point mu and one Dirac pair

[tsondru]

Provenance: audit-sweep phase 1, WI-P07 (N-catgraph-physics-08, minor).

'W1(mu,mu) = 0 for any distribution' is asserted for one uniform 3-point mu; symmetry for one Dirac pair.

Fix: identity/symmetry proptests over random distributions and metrics.

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.