Balance Codex account usage windows

[yvette-carlisle]

Summary

• balance Codex account selection across primary and secondary usage windows
• serialize account-pool selection so equal-capacity concurrent dispatches rotate by least-recently selected account
• update the app-server spec to describe the selector behavior

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make check-vstyle-rust
• cargo make test-rust
• cargo test -p decodex account_candidate_sort -- --nocapture
• cargo test -p decodex account_pool_rotates_equal_full_usage_across_dispatches -- --nocapture