XY-1019 Add Knowledge Workspace version diffs

[yvette-carlisle]

Summary

• add elf.knowledge_page.version_diff/v1 rebuild readback for Knowledge Workspace pages
• expose previous-version diff metadata in page summaries, search items, live adapter artifacts, and benchmark metrics
• document the product boundary and checked-in version-diff benchmark evidence

Validation

• cargo test -p elf-service knowledge::tests::previous_version_diff_records_delta_without_changing_content_hash -- --nocapture
• cargo test -p elf-eval --test real_world_job_benchmark live_knowledge_page_rebuild_lint_has_dedicated_docker_task -- --nocapture
• cargo make real-world-memory-live-knowledge
• cargo test -p elf-eval --test real_world_job_benchmark dreaming_competitor_strength_retest_report_closes_xy955_without_overclaims -- --nocapture
• cargo make check
• decodex docs check
• git diff --check