Implement Stage 12 freshness system

[aarambh-darshan]

Summary

• Add manas-learn freshness APIs for Timeless, Slow, Fast, and Realtime categories
• Detect freshness during teach and stamp source plus freshness metadata onto learned hidden neurons
• Add stale-neuron warnings to Trainer query results and manas ask output
• Preserve freshness metadata through .manas persistence coverage
• Mark Stage 12 complete in README, ROADMAP, ARCHITECTURE, and CHANGELOG

Public API changes

• Add FreshnessCategory and FreshnessWarning exports from manas-learn
• Add detect_freshness, freshness_age_days, is_stale, and staleness_warning helpers
• Add Trainer::learn_with_source_and_freshness while keeping learn_with_source compatible
• Extend QueryResult with freshness_warning: Option

Testing

• cargo fmt --all -- --check
• cargo test -p manas-learn freshness
• cargo test -p manas-learn query
• cargo test -p manas-cli
• cargo test -p manas-store
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --workspace --release