ycb_neon_pr12694

Implement the following change in this repository: [BRC-2951] Enforce PG backpressure parameters at the shard level

Work only inside this repository checkout. Make the code change the task
describes, keeping the diff focused — no drive-by refactors.

When you are done, leave your changes committed or in the working tree;
they are collected automatically.

Reference diff (the task PR's merged diff — the judge's comparison target)

Results across runs

Binary verdicts from the pinned judge (D27). Full attempt detail — candidate diff, transcript, timings — lives on each run page's score matrix.