Design graph-backed rope rebalancing evidence

[flyingrobots]

Problem

HT-0149 defines initial balance invariants but defers the fuller rebalancing policy. The runtime needs explicit structural-maintenance evidence for splits, merges, rotations, and how those facts relate to semantic rewrites.

Acceptance Notes

• Define when replacement triggers balance maintenance.
• Define structural-maintenance fact shape and validation.
• Specify whether and how :why exposes maintenance evidence.
• Link the policy back to HT-0149 and PR Document graph-backed rope runtime gate #205.