Fix hard-disabled enable: BroadcastLivenessHydrator param

[Pitchfork-and-Torch]

Class

7 / hard-disabled enable

What is true

BroadcastLivenessHydrator exists and writes broadcast_is_live, but enable() is a literal false, so live broadcasts never get a liveness bit.

Proof

• Entry: BroadcastLivenessHydrator::hydrate (TES urls + Strato batch_get_broadcast_is_live)
• Sink: PostCandidate.broadcast_is_live consumed by RerankingKafkaSideEffect (no drop filter in this dump)
• Break: enable() ignored query.params and always returned false
• Viewer effect: live X broadcasts in For You never carry broadcast_is_live; ranking/kafka see None
• Twin: TopicFeedbackContextHydrator / EnableTopicFeedbackContext (param-gated enable)

Change

• home-mixer/params/param.rs: EnableBroadcastLivenessHydrator, key rust_home_mixer_enable_broadcast_liveness_hydrator, default false
• home-mixer/candidate_hydrators/broadcast_liveness_hydrator.rs: enable() reads that param
• test: enable_returns_false_by_default

Tests

Added enable_returns_false_by_default.
cargo: cannot run in the public dump

Still open

• Module is still an orphan on main until PR 16 merges (pub mod broadcast_liveness_hydrator).
• Hydrator is still not pushed into any candidate_pipeline hydrators vec. Do not wire until a drop/score sink is named.
• PopularTopicsSource enable() is still a literal false (separate class).