Add reviewable memory summary source trace contract

[yvette-carlisle]

Summary

• add the elf.memory_summary/v1 reviewable summary/source-trace contract
• add a fixture-backed memory_summary benchmark suite with source refs, freshness, rationale, tombstone, superseded, stale, and unsupported-derived coverage
• update real-world memory benchmark reports and the Dreaming readiness ledger to record the new fixture-backed memory-summary/top-of-mind evidence without claiming live managed-memory parity

Verification

• cargo make real-world-memory-summary
• cargo test -p elf-eval --test real_world_job_benchmark memory_summary -- --test-threads=1
• cargo make fmt
• cargo make lint-fix
• cargo make checks
• git diff --check

Decodex recovery note

XY-952 attempts 1-3 retained validated local work but failed to complete Decodex handoff/PR creation. This PR is the manual handoff for that retained worktree.