Bound VM Ranker DPP work by the server pool limit

[garagon]

Summary

Keep per-request DPP work within the pool limit selected when VM Ranker starts.

A RankRequest can provide max_selected_rank. Previously that value replaced the server setting without a ceiling. DPP builds two square matrices from this pool, so work and memory grow quadratically with the accepted value and candidate count.

The request value is now capped at --dpp-max-selected-rank. A request may still select a smaller pool, and zero still means "use the server value." The CLI help now states this contract.

This is a service-level resource bound. It does not assume the gRPC service is publicly reachable; it limits any caller able to submit a RankRequest.

Compatibility

• Values at or below the server limit are unchanged.
• Values above the server limit now use the server limit.
• Home Mixer defaults its request value to 150, while VM Ranker's CLI default is 100. With both source defaults, the effective pool becomes 100. A deployment that intentionally needs 150 can start VM Ranker with --dpp-max-selected-rank 150.

Validation

• The request-to-config test uses u32::MAX and confirms the server limit remains 100.
• A control confirms a request can reduce the pool from 100 to 40.
• A zero-value control confirms the server limit remains unchanged.
• Compiled and ran the exact parameter application function and all three checked-in tests with Rust 2021 in a clean container: 3 passed.
• rustfmt --check passes.

The public repository does not include a build manifest for vm-ranker, so a full service build is not available from this checkout.