Slice: Enforce rope balance and leaf-size invariants

[flyingrobots]

Goal

Prevent path-copy edits from degrading the rope into a linear structure.

Acceptance criteria

• Leaf size min/max and target size are documented.
• Branch height/weight invariants are validated.
• Rebalancing emits structural maintenance evidence without pretending text changed.

[coderabbitai]

🔗 Related PRs

#205 - Document graph-backed rope runtime gate [merged]

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