[v0.2] Phase 6: JSON persistence

[bsesic]

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

Implements Task 20 of the v0.2 implementation plan: a dedicated `tracealign.io.multi_result` module mirroring v0.1's `tracealign.io.result` but for `MultiAlignmentResult`.

Plan tasks

• Task 20 — `io/multi_result.py` with `dumps` / `loads` / `dump` / `load`

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`