DADBEAR unit19: retire staleness queue API

[adamlevineagent]

Summary

• Make staleness::propagate_staleness the side-effect-free deterministic TRACE algorithm the unified pipeline keeps.
• Remove the staleness.rs queue/dequeue API (process_staleness_queue) so this module no longer writes or drains pyramid_staleness_queue.
• Keep a temporary trace_staleness alias for existing unified call sites while tests now assert no queue or source-delta cursor side effects.

Scope notes

• This does not drop the DB table yet: supersession.rs::record_supersession still has a live db::enqueue_staleness caller, so table deletion is a later unit or blocker decision.
• This does not touch Blue/co-owned cascade write helpers.

Verification

• cargo test --lib staleness::tests -- --nocapture
• cargo check --lib
• cargo test --lib edge_survives_endpoint_supersession -- --nocapture
• cargo test --lib faq_link_survives_related_node_supersession -- --nocapture
• cargo test --lib targeted_evidence_resolves_real_source_not_placeholder -- --nocapture
• cargo test --lib stale_check_decision_ -- --nocapture
• cargo test --lib test_set_file_content_hash_preserves_node_ids_order_independent -- --nocapture
• cargo test --lib not_stale_pass_naturally_terminates_without_execute -- --nocapture
• cargo test --lib skip_abstain_naturally_terminates_without_execute -- --nocapture
• git diff --check