Declare ranker Phoenix xDS retry/fallback params

[Pitchfork-and-Torch]

Summary

Ranker xDS dispatch uses feature-switch keys rust_home_mixer_phoenix_xds_max_retries and rust_home_mixer_phoenix_enable_fallback, but param.rs only declared the retrieval twins (..._xds_retrieval_max_retries / ..._retrieval_enable_fallback). README says this file is the cron-synced production defaults.

Adds the ranker params with the same defaults as retrieval (0 / false). No pipeline behavior change beyond documenting the keys.

Test plan

• Keys in param.rs match PredictionDispatch string keys in phoenix_candidate_pipeline.rs and phoenix_scores_pipeline.rs

[Pitchfork-and-Torch]

Parking this intern-stack PR per operator GitHub cleanup (2026-09-08). Hunt notes remain local. Not a reject of the class.