Fix hard-disabled enable: PopularTopicsSource param

[Pitchfork-and-Torch]

Class

7 / hard-disabled enable

What is true

PopularTopicsSource can fetch topical popular tweets from Strato and tag them ForYouPopularTopics, but enable() is a literal false, so the source never runs.

Proof

• Entry: PopularTopicsSource::source (Strato get_popular_topic_tweets, served_type ForYouPopularTopics)
• Sink: PopularTopicsAuthorDedupFilter keeps first popular post per author; Phoenix/For You source stage would consume the vec if the source were enabled and boxed
• Break: enable() ignored query.params and always returned false
• Viewer effect: For You never receives this source's news/sports/tech/etc popular-topic candidates
• Twin: TweetMixerSource / EnableTweetMixerSource (param-gated, skip in-network and cached-posts)

Change

• home-mixer/params/param.rs: EnablePopularTopicsSource, key rust_home_mixer_enable_popular_topics_source, default false
• home-mixer/sources/popular_topics_source.rs: enable() reads that param and clones TweetMixer in-network / cached-posts guards
• tests: enable_returns_false_by_default, enable_returns_false_when_in_network_only

Tests

Added the two enable tests above.
cargo: cannot run in the public dump

Still open

• Module is still an orphan on main until PR 2 merges (pub mod popular_topics_source).
• Source is still not boxed into any candidate_pipeline sources vec. Do not wire in this PR.
• PopularTopicsAuthorDedupFilter is also still unregistered on main until PR 2.
• BroadcastLiveness enable is a separate class (PR 29).