Implement Stage 13 real demo

[aarambh-darshan]

Summary

• add bound hidden-neuron fact storage and best-hidden readout so Stage 13 answers can come from neural weights only
• add decode-friendly answer vectors, bound-memory query variants, and safer CLI fact extraction for multi-relation sentences
• add executable ./demo.sh plus a Stage 13 CLI integration test for the 22-fact demo and no-sidecar proof
• update README, roadmap, changelog, and architecture docs for Stage 13 completion and Stage 14 next steps

Verification

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

Demo Result

• answers are reported from neural weights after deleting sidecars
• brain.manas size: 82716 bytes, below the 500KB Stage 13 limit
• demo.sh is committed with executable mode 100755