Add knowledge changed-source rebuild and proposal flow

[yvette-carlisle]

Summary

• add changed-source Knowledge Workspace rebuild API and service flow
• classify changed, unchanged, stale, and blocked page/section outputs
• queue reviewable memory candidates through consolidation proposal contracts with source refs and reasons
• document the watch/rebuild contract and drift audit

Verification

• cargo test -p elf-service watch_rebuild -- --nocapture
• cargo test -p elf-service memory_candidate -- --nocapture
• cargo make check

Authority: XY-1065