Implement Stage 14 diagnostics commands

[aarambh-darshan]

Summary

• add manas-learn diagnostics models for brain summaries, neuron rows, filters, and query traces
• expose .manas header metadata without changing the version 2 binary format
• expand manas inspect with Brain, Network, Learning, Freshness, Sources, and Layers sections
• add manas neurons with --protection and --source filters
• add manas trace [--limit N] with query variants, selected neuron, top hidden activations, top output values, and final neural-weight answer
• update README, roadmap, changelog, and architecture docs to mark Stage 14 complete and Stage 15 next

Verification

• ./demo.sh
• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo build --workspace --release

Demo Result

• Stage 13 demo still answers all five questions from neural weights
• brain.manas remains 82716 bytes, below the 500KB gate
• expanded inspect output reports Brain, Network, Learning, Freshness, Sources, and Layers sections