Fix dashboard model time progress reuse

[yvette-carlisle]

Summary

• add stable render keys to child-agent bucket rows in the operator dashboard
• prevent bucket reordering from reusing another row's progress bar width during live time updates
• lock the dashboard HTML contract in adjacent operator dashboard tests

Verification

• cargo test -p decodex operator_dashboard
• cargo make fmt-rust-check
• cargo make check-vstyle-rust
• git diff --check
• mock dashboard browser smoke: Model bucket rendered at 83% with no console errors