Design graph-backed rope retention budget dashboard

[flyingrobots]

Problem

HT-0149 requires retention to be measurable, but the dashboard is deferred. The runtime and UI need a way to expose authoritative blob bytes, rope node counts, rewrite/diff facts, materialized cache bytes, checkpoints, and cold-retained blob posture.

Acceptance Notes

• Define metrics and their authoritative source.
• Distinguish evidence bytes from rebuildable cache bytes.
• Add witnesses that catch O(buffer size x edit count) retention regressions.
• Link the policy back to HT-0149 and PR Document graph-backed rope runtime gate #205.