[v0.2] Phase 3: UPGMA guide tree

[bsesic]

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

Implements Tasks 9–10 of the v0.2 implementation plan: UPGMA tree construction with deterministic tie-breaking on sorted witness IDs, plus the post-order traversal helper that progressive_merge consumes.

Plan tasks

• Task 9 — UPGMA construction with deterministic tie-breaking
• Task 10 — `post_order_witness_ids` traversal helper

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`