[v0.2] Phase 2: Pairwise distance matrix

[bsesic]

Sub-issue of the v0.2 multi-witness alignment epic.

Implements Task 8 of the v0.2 implementation plan: pairwise distances via v0.1 `align()`, with canonical (sorted) witness ordering for determinism.

Plan tasks

• Task 8 — `pairwise_distances` function

References

• Plan: `docs/superpowers/plans/2026-05-22-trace-v0.2-multi-witness.md`
• Spec: `docs/superpowers/specs/2026-05-21-trace-v0.2-multi-witness-design.md`