chore(ranking_scorer): add negative weight assertions and clarify quote-tweet boost eligibility

[SB2318]

Description

Fixes #196

Thank you for open-sourcing the repository! While reviewing ranking_scorer.rs and param.rs, I have analyzed five feedback items regarding parameter configurations and scoring logic:

1. param.rs Truncation Check: Verified that param.rs is fully closed and intact through line 1173 (PhoenixScoresResultSize).
2. pdwell Key Mapping: Confirmed that mapping "pdwell" to post_unexplored in applied_weights_map is intentional and consistent with side-effect scoring maps across the pipeline.
3. Negative-Action Weights Sign Guard: Added a debug_assert! check in recompute_sums() to guarantee negative action weights (NotInterestedWeight, BlockAuthorWeight, etc.) remain non-positive magnitudes to prevent sign inversion misconfigurations.
4. ShadowTrafficPhoenixClusterRates Typing: Confirmed Vec<String> is required to encode "ClusterName:Rate" key-value pairs for per-cluster rate overrides.
5. Bidirectional Boost & Quote-Tweets: Added inline documentation to bidirectional_boost_eligible() clarifying that quote-tweets are intentionally eligible because they contain new original commentary authored by the mutual-follow user.

Changes Made

• home-mixer/scorers/ranking_scorer.rs:
  • Added non-positive assertion guard for negative-action weights in recompute_sums().
  • Added clarifying doc comments to bidirectional_boost_eligible() for quote-tweet handling.

Checklist

• Code changes compile cleanly.
• Sign convention safety guard added for scoring weights.
• Code comments updated for architectural context.